Filling the tanks should be easy. When the fire department shows up with a tanker, they should have a pump that will provide more than enough pressure to push the water uphill. Remember that it can take 2 or 3 men to hold a hose against the pressure coming out of it! If you hire a pool filling company, they may count on simple gravity flow to get the water out of their truck and into a pool, but there should be enough elevation to take care of your situation. If you want to go with this route, you may consider an extra fill-only point at the highest tank in case you run into trouble.1 Put a supply/fill point at the highest tank and start pumping in. Open all the valves to let the water flow into the other tanks. The lowest tank will fill first. Depending on how well sealed it is, either let the water backup into the higher tanks, or shut off the valve to the lower tank once it's full.
The emptying is a bit more complex, but should be doable:
- The highest tank will simply gravity feed, no problem.
- The "ground level" tank should gravity feed as the exit point should be at the bottom of the tank, and as the FD is sucking water out of it, it should suck through the lines and up whatever amount of plumbing is above the exit point. There may come a point when there's not enough water pressure to get the last few gallons out of the tank.
- The "below grade" tank will be the difficult one.
- I'd plumb the exit point on the bottom of the lowest tank.
- The pressure of the water sitting above this point should push the water down the pipe and up hill, even if the connection point is above the highest point of the tank.
- Once the water is flowing, siphon action should keep it flowing even without a pump pulling it. With the FD's pumps pulling the water, I wouldn't think you'd have any issue with the water flow.
When it comes to emptying the tanks, if you open all the valves at once, the tanks will (after a few moments of reasoning, but with no scientific method applied) empty roughly in the highest to lowest order, simply based on gravity. However, once a tank is empty, the FD will certainly be sucking air from that tank until they start to draw enough of a vaccume that either they're sucking air through leaks or there's enough low pressure that it's easier to pull water from the other sources. However, that could draw air into their pumps/hoses causing a pause in the water supply at a least, and possibly cavitation in the pump and loss of suction on the siphon from the lower tank (essentially cutting off supply, at least temporarily), at worst. I would think that you'd want (them) to manually coordinate the valve operation to ensure these situations don't happen.
Most importantly, I'd discuss it with the FD while they are there under calm circumstances filling your tanks up, to ensure that they're familiar with the set up and have them recommend a procedure. Put the procedure in writing, maybe ask them to print it on FD letterhead (get a fire marshal/chief or other "official" signature on it), and put a laminated copy in an easily accessible and visible position at the connection point so that if you're not there to direct operations, the firefighters on scene will know that they're looking at a department approved method of operation. Remember, staffing may change, you may be out of town, or even worse, may be incapacitated by an accident or fire itself. You don't want them to rely on memory of a one-off procedure (since your setup will be different than your neighbors) when they're under stress trying to save your property.
1My initial understanding was that the fire department would show up with a tanker to fill this system. It was later clarified by the OP that he is responsible for filling it himself. Since the OP may not have sufficient pumping equipment to push enough water uphill, I changed that part of the answer.
