The closest you will find that may allow long term is Cherry Hill Campground. But the drive from College Park to Rockville is very traffic heavy. There are a few trailer parks in the area north west of Rockville, but all of the remaining RV campgrounds either have stay restrictions for 14-20 days or are at least an hours drive from Rockville without traffic.
I would consider finding a room to rent, it will be cheaper and finding something short term might work out better. When I have had to do that for contracting work before I had my RV, I have luck with roomates.net and while less luck, I still had some with craigslist.
Its a very college heavy are and he may even have luck finding a student house that might need a roomate for the summer.