If you decide on the class C or A make sure and tow a vehicle with you. It sucks setting up camp and then having to unhook everything, put the slides in, clean up for travel only to go an hour into the park and then resetting up when you return. And most travelers I talk to with drivable RV's say the ride and having the slides in makes the room and relaxation a moot point.
West Texas in spring is windy as all get out. We towed a 26' travel trailer to Big Bend last spring break with my F250 and was still moved around quite a bit in that wind. You're basically pulling an aluminum sail behind you so the weight isn't the issue at 8000 pounds it's that trailer wagging the dog.
All that said my family of 5 in a four door F250 had a blast. Stopped in Monahans for one night(which is what a ton of RV'ers did) and then down to Big Bend. Kids loved the Sand Dunes more than Big Bend but they're also 7 and under.
Lastly I would book real soon. Spots are almost assuredly filled up inside the park. We camped at Stiwell Store and RV park which is about 8 miles from the North East gate. Very friendly people and there will probably be college kids out there. They didn't party as they were doing Geology stuff. And be prepared to drive no matter where you Camp. That park is absolutely humongous.