The main idea is that it negatively impacts their property values, because a home is not a home in a lot of places in the US. A home is the largest financial investment most will make in their lifetime.
There's tons of crazy stuff a neighbor could do to affect property values, why single this out specifically for that reason? A clean RV sitting on property is a lot less bad than building a mountain of tires in your front yard or having 40 dogs (random examples)
I'm not saying I agree with it or don't, I'm saying that's the common reason you'll hear from any of the HOA types, real estate developers / appraisers, etc.
Ah got it, yeah fair enough that is probably the most common reason for it that I've heard too
Or a tent since you might as well not invest a ton into what you're doing as it's illegal.