Fresh and gray water tank questions.

[dbhosttexas]

So in my pursuit of electrical nirvana, I ended up with a single 400 amp hour battery instead of a pair of 200 amp hour batteries, which means with the spare tire removed from under the van, I will have an empty spot where the spare tire was.

I do have a class IV reciever hitch to install still, and once that is done I am planning on removing the spare tire winch / carrier and opening that space up.

That leaves me with a sizeable open space that I THINK I can use to install a fresh water tank, which would leave the space between the frame rail and sliding door foot well available for a gray water tank, or vice versa.

I have taken multiple CAREFUL measurements and I know I can get a 20 gallon long, and somewhat hangs down further than I would like water tank between the frame rail and footwell. I would like to have at least 16 gallons fresh onboard, and maybe 10 gallon gray water capacity.

My shower is external in an ensuite tent, I capture gray, when I have to or it is a good idea to, in a pop up doggie pool in the shower room.

This puts me in the position to wonder...

#1. What size tank will fit in the spare tire well, assuming I have no conflicts with the exhaust routing of course... I looked previously and seem to recall it runs outside of the frame rail behind the rear axle. What capacity?
#2. How would I go about getting gray water into the tank if I use the one between the frame rails for gray water as the line would have to go UP over the frame and then back down creating a trap of sorts.
#3. How can I get the gray water out of my doggie pool and into the gray tank? I am thinking a Harbor Freight manual fluid transfer pump into the sink drain.
#4. How do I set up gray tank dump? I am not going to be chucking large solids down the gray tank, larger food bits would be screened out just like I do at home. So would a simple 3/4" GHT ball valve and small garden hose suffice?