SoundGuy wrote:
Why bother adding any tanks at all, especially if this is a budget DIY. Invest in a porta potti and you've eliminated the need for a black tank, invest in a couple of inexpensive 7 gal Aquatainers, gang 'em together, and drain your grey water directly into the 'Tainers sitting on the ground - no need for a grey tank. And yes, I did just this with a previous popup camper ... works just fine. :B

If I'm being deadly honest, my wife has some say in what happens, and hates portas and recirculating toilets.
Also, my parents had one in a pop up that got smashed when we were kids. It wasn't a pleasant experience, not even sure how dad pulled off smashing it.