Wives are best friends too.
Not to be flippant, but these are truly the only 5 words in your entire narrative that carry any true weight as to the circumstances and your decision.
If The Wives become involved, your dilemma could turn from a relatively minor matter into a Hellish Nightmare that will make you wish you'd never bought that trailer in the first place.
Trailers are a magnet for borrowers because if you have one, everybody thinks, "There's no reason for me to buy one when I can use his. He doesn't use it all the time. It's just sitting there."
I would let it go, chalk it up to experience and not loan it to him again, even if you have to come up with some kind of bullshit excuse but you have plenty of time to sit around and think of one.
I would be prepared to replace the plywood on the rear ramp. Once that poultry juice soaks into it, even a pressure washer wont' get it out. Once that thing sits closed up in the heat, the stench will return with a vengeance.
In the Grand Scheme Of Things, if he's been a standout guy since you've known him and this is his only intrusion on your sensibilities (although I do consider it quite rude) it's simply not worth stirring up shit over.
Like I always philosophize, "I doubt it's something I'll reflect upon when I'm lying on my Death Bed."
Not excusing his behavior, but he was hauling grills to cook turkeys at a church for Those Less Fortunate, not hauling kegs of beer to some party full of underage drinkers. That's gotta count for something......
Oh, and if I didn't say it before, don't loan it to him again. U-Haul has every reasonable rates on local trailer rentals.
I will say, though, I do understand your frustration. I've loaned movies to friends who were laid up after surgery and never gotten those movies back, given a lot of food to people who never bothered to return the dishes it was cooked in even though they obviously came straight from our kitchen, etc.
I just let it go and remind myself, "Well, at least I'm not like they are."