do you have aux batteries? If so where did you locate them? Your water tank location is exactly where I was planning to locate my batteries.Indeed, but you need to have access hole for gravity water feed which I don't have. In my case the tank is under the floor in the cavity under the sliding door. For the gravity fill method I would need to penetrate the floor, or the upper wall in the sliding door step are have some kind of fixed or swinging u-tube device allowing gravity feed. When my original concept of running a large diameter filler hose from the tank into the engine compartment did not work out I picked the pressure fill method thinking that that it will be the least complex, time will tell. I already have 1/2" shore water hose in the engine compartment so using that hose to for pressure fill method was a natural progression.
Once done, the fill of the fresh water tank will be very easy with just two electrical switches or perhaps even one switch.