[38Chevy454]

I think you may run into conflict with ability to sleep 6 and stay under length limits. But look around, you might find one that can make it work. The problem with converting a dinette or even a couch, is they tend to be kind of short to really stretch out. So the main BR and the overhead bunk are both long enough, but the couch or dinette are not so good for adults.

I suppose you have already been looking on racingjunk.com, that seems to be the best place to see a lot of toterhomes for sale. Your budget is OK, you will be dealing more used and higher miles, but that does not mean used up. Remember that most of these truck conversion chassis are designed for 1 million miles as a class 8 truck. Engines may or may not go that far before some overhaul work; and there will be some chassis work needed during that 1M miles.

As to the engine and trans, I suspect you will have to decide if what you find will work for you and your use. Never heard anyone complain about too much power, but too little can be heard. If your 1-ton can handle the pin weight of your trailer, certainly a single rear axle will be sufficient, even given higher truck and RV portion weight.

I know this is a more complicated solution, but have you though about a longer MH that has a garage as part of the RV box portion? Then get a two-car tag trailer to replace your three-car gooseneck trailer. Once at your location, the vehicle in the garage stays outside and you can use the garage for sleeping area. Lot of toy haulers use fold-down bunks, you could do similar or just an air mattress.