Size black and grey water tanks for an Ford E-150

You are best off with a black tank directly under the toilet, for a straight drop into it.

A lot of van conversions use 5 or 8 gallon jugs in a cabinet under the sink for gray water. You could also extend the sink outlet to go through the floor, and then over to a container set outside.

If you google around you will see a lot of ingenious systems for popups. Some use wheeled totes, some use jugs.