Cunningham Falls State Park is in Thurmont, MD. I've never stayed at the RV park, but I have done day trips there. Beautiful park and close to Frederick. Not sure about 40' sites... might be too much for a state park.
Brunswick Family Campground is in Brunswick, MD on the C&O Canal. Its about 20 minutes outside Frederick on the water. I've never stayed here, heard about it when talking to some people in another nearby park.
Little Bennett Campground in Germantown, MD is about 25 or 30 minutes south of Frederick, right off 270. I've stayed here before and its a nice campground. Again... that 40' trailer is what is making it difficult.
There are several campgrounds in Harpers Ferry, WV, that's about 30 to 40 minutes outside Frederick.
There are more campgrounds in Hagerstown, MD that's about 30 or 45 minutes north of Frederick.
There is nothing that I know of directly inside the city limits though.