Originally Posted by JoleneAS
Are there any options to interlink the tanks under the subfloor or is that less recommended?
Anything is possible with enough money to throw at the problem. You may recall, I have a composting toilet, so I don't actually have a black tank. I have three gray tanks that can be combined for 44+ gallons of gray capacity.
My two 16 gal auxiliary gray tanks are connected together horizontally inside the belly pan with 1-1/2" ABS pipe, and there is a valve between those two tanks and the dump manifold. Close the dump valve, open both my primary and secondary gray valves, and I can use all my gray capacity. Close all the valves, and only my primary tank (which receives all my gray water) will fill up. This is helpful in the Winter because my primary gray tank is heated but the two secondary tanks are not.
If you do link multiple tanks inside the belly pan, it's worthwhile to think through whether you need extra valves to close them off, and where. For instance, if you have a full gray tank around the middle of your rig and your shower is in the back, what happens if you drive uphill? Maybe you want a valve to close that off to prevent the water from backing up into your shower while you're underway. If one tank is heated and the other is not, it would be worthwhile to have a way to direct waste only to the heated tank.