If I had an unlimited amount of time and a good budget, I would love to do something like that for a sema project. Full resto on the truck with the idi conversion and modern suspension/trans, full rebuild of the camper. Maybe one day
An RV is a slightly easier route if you can find one with an IDI already in it. It might get slightly better mpg vs a pickup+slidein, since there's no truck bed weight. RV insurance is much cheaper too, especially if you want something dedicated to camper use.
Unless you are wanting better ground clearance, then a truck is better. My 2WD F250HD (350 rear end) looks 4WD almost ground clearance wise. Of course, 4WD conversion is possible on a van chassis.
The E350 RV's ground clearance is much less, but I an still get under it without lifting it. I am slim however, so bear that in mind. I did the RV rear shocks in a Walmart parking lot several months ago.
If my truck had turbo, I'd have definitely gone with a slide in. I suppose for the money I paid for this stupid RV plus repairs and tires, I could have bought a good used turbo kit for it and a nice big slide in. With the truck's 3.55 it might get slightly better MPG vs the RV's 4.10. The truck's disadvantage is being SRW, so too big of a slide in would kind of suck, unless I put in air bags or something.
But, I do love being able to park and be inside already without having to go outside to go inside. With my dog, that's a big pain in the neck. I did so for a long time but I constantly dreaded it. If it's raining, she will outright refuse to get out. That's why a motorhome appealed so much to be. AFter dealing with a truck camper for a year and a half and a trailer for about 6 months, I decided a motorhome was better for me.
For weekend and casual use though, I'd probably opt for a slide in, if I was removing it from the truck for daily truck use.