IMO, if you want to stay under your truck mfg ratings, you need a 3500 and preferably a dually......even then it may be tough. I'm regularly over GCVWR by 7K WITH a 3500 dually.
If the claimed numbers on the 300x12 are correct, it wouldn't even be on my radar as a toyhauler. There simply isn't enough CCC to do anything with a 12' garage.
For just using the garage as a spare bedroom, staying in a full hookup park, and hauling maybe 1-2 "sport" ATV's it could be OK. Anything more and it will be overloaded.
In my mind, a 12' garage unit should have at least 4-5K+ in CCC to be safely usable for boondocking.