Hi RRoger,
Do not assume that living in an RV even in boondocking mode is inexpensive.
100 gallons of fresh water is a little over 1 hour of "pump" time. That could be extended by using grey water to flush the toilet.
My RV is well set up for boondocking down to about 18 F. Much below that and it is far cheaper to be at a campground than to run the generator.
If it is hot weather--again a cheap campground is far more inexpensive than the generator. A months rental is about $800 in the "only game in town" during the winter months.
When I was part time, I enjoyed the challenge of living frugally. Now that I am full time it is not nearly so much fun.
I believe you are looking at this with rose colored glasses. From what you have posted, so far, it appears you don't have a lot of the technical skills needed to do a conversion from a bare bone box to an RV.
It is nice that you wish to think "out side the box", but there are reasons why even the largest RV's are not longer than 45 feet.
If you have a couple of million to spend, or even $500,000.00 you can DIY yourself anything you want. The electrical system alone is going to be about 20 grand, if you want it to be autonomous and grid independent.
Chassis repairs are rather rarely a topic here. When they are mentioned it is often "operator error" that causes them. That tells me they are infrequent.
While there are many rants about poor design, taking an RV and upgrading systems is going to be far more inexpensive than starting from scratch on a too large 48 foot box van.
My initial purchase price for a 28'5" used (less than one month of use) class C was $37,000.00. I have spent $19,000 on upgrades and improvements. It is finally four seasons capable (even though it was supposed to be from new). It is not economically autonomous from the grid in either summer or winter. I can probably get to that point for summer time, with an additional cost of $4000.00, but in winter time, solar just won't provide enough energy for me to survive off the grid. Running propane heat is prohibitively expensive. I need a source that can provide about 6000 watts peak if space heating is electric.
Even if I, by some "magic" managed to do inexpensive autonomous heating in the winter, I'd still need a way to dump and to take on fresh water. The only year round dump site I've found charges $20.00.
Please carefully rethink your idea of converting a 48 box to an RV.
