The Nuvi units you listed probably are not water resistant.
Not sure if Lifetime Maps is transferrable.
Worse than not water resistant, the nuvi GPSs use mini USB connectors for power. It's not intended as a rugged connection; it was designed for your desktop. Worse, Garmin uses the 5th wire in the (normally 4--wire) connection to signal that it's a power cord and not a data cord.
One day I rode from Maine to PA in occasional showers. The next day, about mid-day my nuvi 1300LM went dark. That evening I tried to charge and get it to work, but nothing. Then at the end of a week, it started working again. That was several years ago and it worked fine until just above Anchorage in dry weather (although only a day or two after some rain). I tried for a day to revive it then bought the Montana.
So you COULD say that my nuvi has performed fine for many many miles and had only 2 failures. But . . .
Lifetime maps are NOT transferable.
As for those gas bars, you can add "waypoints" to any GPS, right on the GPS. The exact method differs, but you'll zoom and pan around until you see where you think the gas bar is. Then tap the spot or a button to add that point as a waypoint and edit the label to say "gas bar".
You don't have to be exact. If you make marks for gas bars in Ft. Liard or Ft. Simpson, you can put them in the wrong place as long as you ride down the main street once you get there. Distance to the waypoint won't be off that far. Or you could just navigate to the towns since there's no way to miss the gas. Same for Ft. Prov.
After Ft. Prov, you may have to do more work with Google maps to place gas bar waypoints. The gas may be down behind the general store and the pumps may not look like what we're used to.
My point is that you don't have to make a map of gas bars in a different program then transfer the map in. That may work fine if you have the time to learn it all.
I just looked at a 2012 Milepost guide to those roads. It says "no services between Ft. Providence and Yellowknife". That's 340 km. When I was there that would be well within a tank for the speeds I rode on the firm roads with the state of tune of my KLR. YMMV or something. I MAY have been able to make the 444 km from Ft. Liard to Ft. Providence without the side trip to Ft. Simpson for gas. I would have been on reserve. I know this thread is about GPS, but you may want to buy a small gas can in Ft. Nelson or Ft. Liard.