As an old aged rider of 85 years and some 600k miles in dirt, mud, and street including racing Motorcross and Enduro, starting in 1954... I too think it ill-advised to suggest the TAT to anyone that doesn't have a good bit of time on a KLR or something similar on dirt, Georgia Red Clay and sand. Note. Rem to deflate your tires down in sand, dirt and or rocks. Gives better grip to tires. Thus the small compressor I carried that plugged into a lighter socket to air back up to street speed on the way back home
I've owned 3 Goldwings, from the first year of 1975 when it was just a raw 4cyl bike with lots of power and terrible shocks up through a 2004 and the lastly to a 2012 that was a super good touring machine. Then my touring went to a used 1999 BMW K1200LT and my eyes were opened to why you spend good money for BMW's. Currently own a 2007 LT with color matched Hannigan Europa Ii trailer. And I've ridden both of those 900+ pound bikes on dirt/clay roads and some sand where you are up on the pegs in 2nd and 3rd gear and keeping the bike up on the sand and NOT down in it. So that's all that.
Now... you can't do better that the KLR650 IMHO Your mileage might vary, and thats perfectly ok. My TAT and TET and One of Four Iron Butts, were both done on a Highly modified KLR-685. Not a typo, 685. Seems that coming back from a week in the wet muck of the National Forrest SW of Tallahassee I seized the engine. I let it cool, 3rd gear, clutch in and 2 big guys pushed me hard as they could and I dumped the clutch. She started and I eased it back home the 90 miles to Valdosta and we boared it out and it came up to 685cc. The suspension was totally replaced, went to a solid handlebar, gell grips, taller windscreen, metal bash plate, engine guards, high grade chain and auto oiler and installed a heavy duty wiring harness to support all the upgraded lights and added night driving lights, GPS, phone plug and air compressor port. Note. The KLR has that big 6,3 gallon tank. In Utah and Col you will love that about the KLR. Also, from personal experience, you will easily find a Kawasaki dealership way before a KTM or BMW. That's a big plus.
Why do I suggest the KLR against a BMW GS or other Expensive ride? See above and, I spent about $6 grand on the new KLR. Spent another $3k on upgrades.
Things to do before doing the TAT. Read everything you can find from one's that rode it. Get your legs in shape. If you have a bicycle, rowing machine, stairmaster etc. Use it or ride it. Get together your tools you are going to carry together. Put your bike on the center stand (yeah get that too, you'll probably change a tube in the rain.) NOW, use only the tools you'll be carrying, remove both tires and change out a tube and get the tire back on and adjusted. Note, carry an extra 19" tube with you. Yes you can stuff that 19" tube in the 17" rear tire, especially if you have a small ZipLoc bag of baby powder. Also from personal exp. Change the oil and the other things as far as maint, on the bike. You're not going to have a took wrench in the TAT.
Purchase the maps and GPS Routes from the pharmacist guy that laid this great ride out. Right now can't remember it with a touch of the brain farts. The Maps and Turn by Turn Roll Chart is almost a necessary in my minds eye. I installed a TrailTech Vapor including the necessary water pump piece that let the Vapor monitor the temp. The Vapor also ran a very accurate Odometer that will read in hundreds of a mile. Over kill you say. Well there are several places where you come out of the woods and run 100 feet and back into the woods and a trail is almost invisible.
Go to Tractor Supply or the likes and get a water proof PVC instruction manual/ document holder. About 11" long with screw off top. You can get All your tools that are necessary to fit into it. Using large enough clamps, clamp it to the frame on left side just in front of your Canvas saddle bags. Canvas saddle bags, WTF on my new TAT bike. YEAH SOFT UGLY BAGS. Square metal bags you say. Lots of $$ and when you go down the brackets bend and the sharp metal corners make a Hell of a place on the back of your leg, and hit you right above your boot tops. Leaves a Mark.. You will also go down in sand someplace where it's really hard to get the heavy beast up. Cloth bags Un-strap easily incl the top bag and with that and your large yellow dry bag with your inflatable mattress and sleeping bag and changes of clothes and the likes... you've suddenly made the beast 70 lb lighter.
Go with a large friend, one like I had. Kenny had money and when I convinced him this was a ride we needed to do, couple days later he had a new KLR an set his up nice too. We had Senas in the helmets too. Kenny picked my butt up many times.
All of this happened shortly after 1980. My best friend and my family doctor spent the night in a motel in Port Orford, OR on a So. GA to Cal trip. Gone about 30 days. I was on the 1975 GL, heavily modified then for LD touring. We got up to cross the road for breakfast and noted all the decked out AT bikes that had come in over night to the motel. I went down the line talking to them thinking I'd done something... riding a big easy chair on pavement all the way to California. I was embarrassed shortly, but vowed I was gonna have me a KLR650 and do 'The Trail'.
Well, I did, and I didn't. Made it into the neck of Western OK the last trip and it was Hell. It had rained for a couple of weeks and the square designed roads in North OK, go a mile, turn and go two miles and turn were gummy. We were told that petroleum products were mixed into the roads when they were made and too much rain and we slid all over. We found a truck wash in some cross roads and I think I spent almost $15 in Quarters getting that mud out from under my bike.
My buddy had to return to work so we came home. Time passed, I got older and shortly after getting back from a 27 state around the edge of the US, 31 day tour with my Doc friend and 4 others, again a different Cal treck; My sweet wife and a nurse, informed me that the Arteriogram that I pushed back to make the 27 state treck, was way passed due, so shortly I awoke, becoming aware of my worried wife and a different second doctor looking at me with concerned eyes. Good and Bad news: I had sneaked by having the massive killer heart attack by weeks. And two days later they went in and did 4 by-passes on me. Saved my life there. On my 6th week visit with Dr. Gajohnsonn, who was from Iceland, told me I was doing better than most my age and were getting great reports from the therapy girls, I asked him, when could I drive again? His comment was soon as I had quit the heavy pain meds and with Sharon, my wife on board. I related to him I'd been off meds over a week. So he says this coming week. He knew by now I was a motorcyclist- Not a biker!! I have no tatoos and no metal in my outer skin, lots in various bones tho. Sooo My next question to him was, When can I Ride?! Ohhh he went off on a rant about the dangers of motorcycles and he didn't want me on a bike until the new year. It's September then. He had been a Naval Commander and had just chewed me out. I hesitated a bit as he took another sip of his coffee standing there in the door way to his office. I finally said, "Dr Gajohnsonn, I've been a bad boy!" His face took on a bad look. I said, " I rode my Gold Wing down to this appointment. I thought he was gonna' hit me with his coffee cup. What I got then, is something we in the south call, a 'Scientific Cussing!"
But still here, Thank You Lord.