I currently live pretty close to Flemington. It's a nice are, but not a campground type of place. I've never heard of the Mountianview cg. Not that I know every campground in the state, but I would have thought I'd heard about it living so close. If you consider that cg, don't be tempted to take 579 to Flemington. There are two killer hills with stop signs at the bottom that I wouldn't want to take a MH down.
There is a cg on rt173 off of exti 11 of rt78 called Jugtown Mt cg. It doesn't get real good reviews, but I thought I'd throw it out there.
Your best bet is Spruce Run. It is right off of rt31, which you will probably take into Flemington anyway. It is not full hookup, but does have water and a dump station. Being on a reservoir it is pretty nice. Just don't bring adult beverages, they're not allowed. I'd say it is about 20. Minutes from where you have to go. The Allentown KOA is probably more like an hour away.