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