Personally, if mounting water tanks high, I'd be concerned about center of gravity issues (full 55 gallon drums weigh around 400 lbs. each). The lower the better. I mounted a 55 gallon drum under my floor just behind the differential (using the diff as an impact guard of sorts).
If you're trying to use gravity as a means to pressure your system, at 7' don't bother. While it would drain at ground level (who uses water at ground level except for irrigation?), you only get .433 psi per foot of elevation. do the math and you only get a little over 3psi at the ground. My suggestion would be to mount your tanks low and use a pump to make things flow through the pipes. Fortunately, on a skoolie, you don't have to lift the water very high.
And yes, you can tie them together, you just have to make sure both vented separately to fill and empty with ease. Connect both at the bottom and vent both at the top. Fill the top one from the top and you're good to go.
Hope this helps.
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