I think this is an interesting discussion and would like to take a run at the question: is it better to have a newer, high-mileage vehicle or an older, low mileage one.
I think the answer is, it depends.
Some things fail from age regardless of the miles. For example, rubber parts like belts, tires, hoses, weatherstripping, etc, can rot from age even if the van hasn't been driven. Furthermore, if it was in a wet climate, there may be leaks, mold and rust; if it was in a hot and dry climate, the wiring harness may be cracked and brittle. So for an older van, being stored inside is a huge bonus and would add value.
"Regularly maintained" is kind of a loose term. The engine oil may have been changed regularly, but the owner probably didn't take the door panels off and re-lubricate the window motors so there may be some age-related issues with non-drive train parts.
On the other hand, a newer, high-mileage vehicle is more likely to have worn ball-joints, bearings, etc. It may look newer, but perhaps require more expensive repairs. Furthermore, being driven between Florida and South Carolina will have put far less stress on the mechanicals than being driven around the Sierras or Rockies. So knowing where and how it was used would be helpful.
It also matters "when" it was driven. If you find an older, low mileage vehicle, but the miles were all put on in the first two years and then it sat idle for 10 years, there could be issues. Mechanical items need to be worked periodically.
I think the repairs you will need to make based on age will be less expensive than the repairs that would be required from use so speaking purely from a chassis view, in an ideal world, I would look for an older, low mileage vehicle that was stored inside and check the Carfax to see if it was driven at least a few hundred miles each year.
The above is just my opinion so YMMV, as they say.
I can't speak about the RV "innards" but there are a lot of people on this forum who know more than I do about these things.