I would not worry about a gravity drain water system. If the tank is 10 feet above your area that you plan on using the water, it will provide just 5 PSI to the system, and not enough pressure to get through the piping and come out the shower head.
The water pump is very energy efficient. IT will pump 2 GPM for 60 minutes using just 7 amp hours of power. In other words 120 gallons of water for 7 AH or 84 watts.
I probably would be looking at other RV's that have been scrapped, and picking up a couple more fresh water tanks. Then fill them and take them back to the camper when you are going camping.
If you want new tanks, this is one location.
Tank-Depot.comGood luck,
Fred.