All roads mentioned are good for big RVs.
No charge for driving the highway through the reservation but if you want to tour Monument Valley, yes, there's a fee.
Also, if you're planning to be around Page, AZ south from Page on Hwy 89 was a collasped highway just before 89A and there would be a detour but I think it was scheduled to be back in operation this fall. Try Googling Hwy 89, Page for an update. From 89A/89 intersection, Hwy 89 into Flagstaff is very good.
Water: We always carry water with us - just in case - and it doesn't affect fuel mileage. But if you want to wait, why don't you just get it in Moab? There are plenty of RV parks that would let you get some. Where are you planning to stay in Moab?
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel