A $ 1,500 shipping container, and some material... who knows,, maybe $ 15,000 ? $ 20,000 ?
A starter project ?
Check out the prices of gas motor homes.
I have a classic 1978 Travco and it has provided 35,000 of delightful travel to such places as the Skyline Drive to dirt roads leading to a friends home in the Panhandle where he was divinf gor old growth sinker logs.
I used a Mercedes 300TDT for a toad. I just let it idle while pulling it and the trannie never gave me the first bit of grief in those 35,000 miles.
Having managed to control my DIY urge to mor a SS GMC bus, trust me when I say an existing RV will be less expensive and allow you to enjoy RV life rather than spend time and money on a project which will most likely not bring any where what you put into it.
Selling an older RV can allow recovery of much of the original price you paid for it
Box trucks by Ford are about $3000 around here. Not new ones of course but in the 1990s era and a box truck would be very fix up able in a pinch. I think.