My 2014 used 18.5 qts after oil and filter change. I weighed the coach, went to the tire manufactures web site, used the weight table and now run 85PSI front / rear (that's with a 10% fudge factor). Flat towing a toad doesn't affect tire loading (weight rating) like towing a trailer with added tongue weight.
I added "Horn Blasters" Katrina train horn system. The system has its' own Viair compressor and 5-gallon storage tank. Horn's are unbelievably loud, I didn't want to tie into the factory air system for safety reasons.