Gumpy I have a question you said (due to the way the upper tanks were plumbed).
What happened? Because rigid piping?
Do you need a flex joint? Expansion joint ECT?
I have a smaller tank on each side of the bay, mounted in the area between the tunnel and the door. I initially connected them to the main tank on the floor using 1/2" PEX. The idea was that as the lower tank filled, it would displace air up through the upper tanks and out their vents. When the lower tank was full, water would then fill the upper tanks through the connection lines. When they were full, water would run out the vent tubes and I'd shut it off. There wasn't enough difference in height to be a problem with pressure.
And it did work, for the most part, but the problem was that the 1/2" PEX was just too small, and trying to push water through those lines with city water pressure caused the top of the main tank to bulge with the pressure. I had seen this, and usually throttled down the fill when it started filling the upper tanks. This time, though, I was busy doing something else. When I looked in on it, I saw the top of the main tank really bulging, and reached for the shutoff valve, but before I could get it, the top seam blew out in the middle of the tank. Just popped it, and squirted water all over. I replumbed the upper tanks so I could still use them separate from the lower tank and we continued on our trip. I've now changed the plumbing permanently so the upper tanks are filled separately from the lower tank. Now, I use the water in the lower tank first, till it gets down about half way. Then I open the valve and let the upper tanks drain into the lower tank. I just have to remember to shut the valve when the upper tanks are empty, or my pump draws air from their lines. Not quite as convenient as before, but easier than trying to change the connection lines to 3/4", since I'd have to remove my black tank drain lines to get the upper tanks out.