Beauregard, welcome back to our website and forums. We are glad to have you become an active participant. I would recommend that this type of issue be placed in the Newmar Owners forum as it is a more specific type question for your unit but other units are of similar design.
By the way, you will have to access the valves by dropping the belly. Once you know exactly where the valves are located and while you have the belly down, you could install some framing to attach an access door for future needs. Good luck and keep us posted.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7