This question has been asked and answered many times. RVS ARE ABLE TO USE THE ENTIRE LENGTH OF THE GSP.
I see them using it all the time. The restriction is for commercial traffic only. I have a link to the toll rates you will pay which include your vehicle class north of Mile 105. The other link is to another site answering your question.
http://www.state.nj.us/turnpike/documents/2012parkwaytollrates.pdf
http://www.irv2.com/forums/f42/nj-garden-state-parkway-re-5th-wheel-204826.html
I live 20 miles northeast of Branchville and have RVd this entire area. The best way with the least hassle is:
GSP North To
Route 440 West which turns into Interstate 287 North To
Interstate 80 West TO
Route 15 North
Alfred May
2005 Excursion V10 4.30 4x4
2002 Cedar Creek 30RBS TT by Forest River
Reese Dual Cam
Tekonsha Prodigy