RSchleder summed it up well. Here's the post he was referring to: http://www.classbforum.com/phpBB2/vi...&t=2663#p15700
It is worth taking an older van to a mechanic.
For the RV side of things it should be demonstrated to you by the seller that everything works. I didn't test all systems when I bought my current van and right away it needed a new water heater (maybe $400 from memory) and new tires at a cost of around $1,000. I did get the van for a good price and didn't really mind getting the new parts but my point is that it is a good idea to have some money set aside or at least allowed for unexpected repairs to an older vehicle when negotiating the purchase price.