In an RV, it is clear that someone is sleeping there. Someone who would be better off staying in one of the hotels and spending money. There are some places where such vehicles are associated with prostitution, or other "undesirables" like gypsies.

Some beaches explicitly prohibit RVs, or put gates that make it impossible for such large vehicles to pass.

Campervans are less problematic, because unless they are heavily customized they look like regular vans, so you can park almost anywhere without raising too many eyebrows. They also tend to have a smaller profile, which helps with road restrictions.

My parents toured Europe several times and they use an almost stock white cargo van for the reasons I stated above.