Most campsites in Nickerson are fairly accessible and fairly flat ... there are quite a few tho that have some dips and grade to them. I only know of a few that are too tough to use with a TT ... sorry I can't name those.
IF you call the rangers at the Brewster facility, they will let you know which sites are good for your camping vehicle. (508) 896-3491 Direct to park ranger
You can also see sites on the Mass State Parks website.
Nickerson is a great park - lots of activities -- awesome bike trails .. no HU, but water and bathhouse throughout park, also a RV dump with four stations
Monkey44
Cape Cod Ma & Central Fla
Chevy 2500HD 4x4 DC-SB
2008 Lance 845
Back-country camping fanatic