This came as a surprise

Very few municipalities have laws that outright ban personal trailers or RVs from parking in the street just for this specific reason. There are people who own them and need to do what some posted (i.e. park it, then go get supplies to load up) and in my experience, most of them make some provision for this. (Usually for a limited number of hours).

In the OP's case, I'd like to know what, specifically, the law he was cited for breaking states. Even though it's only $42, if I was in the right, I'd sure as heck fight it!!