We have considered and stayed in tent sites when nothing else was available, but the sites are highly variable. Some have group parking with walkin to the sites of various distances, some have parking that is very unlevel and unsuited to a van that you stay in. Some even have rules about sleeping in the vehicle. We have a 20' Roadtrek Chevy 190. What you don't ever really want to do, in courtesy to tenters is run the engine for power or lights or the generator unless it is in a generators allowed tent site. We have never been asked how long we are, although they may look at the van.
Even an 18" van will be tough in the city, kind of like have an 18" pickup truck. Park at the end of blocks so you can't get parked in, etc. In the towns that have nose in parking, 18' would be an advantage to keep the back end from hanging out in traffic, although these days there are usually pickups that big there also. 24' would be out of the question there, I think.
Personally, I would look at what you need inside to decide, not length, if you are under 21'. An 18' B can be fine for a single traveler, but tough for two or more people. Smaller vans often have crosswise sleeping which for more than one person may be an issue. Front sleepers are often very good for single travelers.
Good luck in you hunt!