Well I thought I would close out this thread.
I finally finished the plumbing installation enough to test the grey water system.
The tank I had made ended up being around 27 gallons. I mounted it below the floor just aft of the rear axle. It's 4" deep so it will be hidden in the bellypan.
The tub and bath sink drains tie in together below the floor in the rear and run back into the side of the tank.
The kitchen sink drain runs to a tee, where it goes to a roof vent, and the other part of the tee goes into the top of the tank under the street side bed.
I was able to dump at least 25 gallons of water into the kitchen sink until it backed up into the tub. The tubs drain/trap always has water visible a couple of inches below the top of the drain. Never rises out of it unless the tank is over filled.
I started draining the tank and when the water in the tub went down, I used the shower to rise out the tub. The drain kept up with the shower water just fine, even though the tank was so full, the side inlet was under water. So it still drained.
I'm pleased this has all seemed to work out so well. Of course it has not hit the road yet with the sloshing and bouncing.
Here is a photo of the grey tank. The two rubber couplers were used to allow some flexing. One is the inlet from the tub/sink, and the other is the outlet to the dump valve.
Thanks to everyone for the help with this unusual science of grey-tank-ology!