I agree with Belegedhel that each tank needs to be vented. You won't always be perfected level side to side/front to back. (lesson learned the hard way). In a perfect world you should vent at opposite ends and opposite sides (i.e. front tank front corner curb side; rear tank rear corner street side). That isn't always practical, but something to consider.
There are flush mount spin weld fittings available, but being recessed means they won't allow a tank to completely fill. You have to drill a hole through your subfloor anyway, so it has to be a little larger with protruding fittings.
https://www.vintagetrailersupply.com...tings-vts-344/
You can have VTS mount the fittings for you, but make very accurate measurements of where the fittings will be not only on the tank, but where they line up in your subfloor (under the cabinet, not in the aisle). They are not hard to install with a good router and the proper driver.
I would also suggest having a valve between the gray tanks so water doesn't migrate back and forth while traveling. I used a cable operated valterra valve that is located on the shelf of the kitchen sink cab.
Pull up to open down to close.