There is a reason modern RVs tend to have attributes you have commented on. Its to save weight and while the skoolie will be cheap to buy if you DIY the mods, you will end up with a really heavy rig and that means you will pay for it in performance and fuel mileage in the long run. Maybe there are some out there that are better but all the buses I've experienced had really lack luster performance- I would never be happy towing a Jeep up WV mountains in one loaded with an RV interior and supplies. A 1 ton Van(with towing package including the biggest 6.0 motor) with 4500lb travel trailer(off road setup) gives us storage and off road capability up to the point where 4wd is needed. We pass car climbing mountains. At under 25' we fit in any campsite and have a good setup to go on day trips for biking/canoeing/whatever while having a completely setup homebase to return to. Pretty much everything is a compromise when you are using a vehicle for multiple purposes so you are the only one who can evaluate how the pros and cons will work for you. But a few other considerations- factory made RVs have predictable values so you can know going in how much you pay to try it and bail if its not working. If you buy a used RV you are missing that initial big hit of depreciation and the depreciation slope is then much lower.
School buses are not designed for off road use for the most part nor are they in the same class as light trucks and cars as far as maintenance goes- so I hope you have good info as to which ones to buy to avoid long term issues. Again, you are the only one who can decide but for me, any rig of an integrated motor coach design is more suited to glamping rather than go anywhere, do anything camping.