I don't know how much a compressor off a big rig pushes for air... but assuming it's enough for what you need....try this for an idea.
Try connecting 2 or 3 tanks with a common supply line from the pump. Since your pump is below your tank, make sure you have a drip leg to hold any condensation from that line that will get it below the pump, it wouldn't like moisture draining back into it.
Plumb fairly large connections to equalize air between the tanks so that you only need one take-off from one tank to your regulator. Pipe is cheaper than hardware.
Plumb your shop air connection from the top of your tank down to your regulator, where you can get to it, and again make sure you have a drip leg below it in that part of the system. Your air take-offs should come up from your supply.
Put a drain line from the bottom of each tank to one common drain valve down where you can reach it. If you keep the plumbing compact enough in geography you may be able to get the drip legs and the tank drain plumbed together to one common drain valve.
Becuase your regulator and compressor pump are below the tank you would have to be religious about draining, and even then I am a little nervous about that layout.
I recall a good shop air thread a little while ago, that may give you some more ideas.
Check this out, very comprehensive
It's referenced in earlier threads.