Everyone thanks for all the advice. We just got back and the trip went great! The RV let us enjoy the trip despite the 100+ temps.
The first 30gals of fresh water lasted until final wash-up before heading home. The 36/36 grey and black lasted us the whole trip with 3 people using the toilet (youngest 2 in diapers).
The RV was kinda cramped for a week and the slide blew a fuse each time we operated it. Biggest issues spacewise were top bunk not reachable by my kids and dinnette sized for midgets. I think we will rent a ~26' non slider next time.