One other thing to think about is a holding tank thats 4" deep unless very ,small like 4-5 gal would be very limited when just slightly out of level
There are holding tanks made that are around 20 gal and 5" tall
If that little depth was used for fresh water and got a little out of level the wrong way you would just suck air
I added 23 gal fresh water cap by teeing into the line before the pump and adding valves and a exterior line with a kwick connect by the camper elec harness . 2 saddle type tanks fit in the storage and dead space in front and behind to wheel wells of the truck. Vented together they fill with one fill hose acsessed at the gap btwn camper overhang and truck bed side.
Flip the valve on the stock tank off and the truck bed tanks on and the pump pulls water
Also when off the truck with a short hose I can pull water out of any tank or bucket