Don't by a new RV!
Purchasing a Brand New RV is an incredibly bad idea. Not only do they depreciate like a rock, but even brand new they typically aren't built very well. This has gotten even worse in the last couple of years. To make it worse, the sort of “lemon” and consumer-protection laws that protect you from bad cars typically don't cover RVs. Here's an excellent video from a consumer protection attorney explaining why.
There is some debate about the value of extended warranties.
Buying a new van is a slightly different story. It's still not a great financial decision (especially if you're financing), but they don't have the precipitous deprecation that RVs do. However, you can still expect to lose 50% of the value of a new vehicle within the first four years.
For some people, this loss of value is an acceptable trade-off if it means they get exactly the van that they want (especially if they have some specialized or esoteric configuration options). However, unless you are someone to whom losing ~$6,000/year is acceptable, buying used (even slightly used) is almost always the better chose.
The sweet spot for buying a used RV or Van is typically 3-4 years old. You've let someone else take the biggest chunk of depreciation hit, but you're still enjoying a modern vehicle that has likely good reliability.