Evy,
Your level indicator will work, but the bottom connection should be made at the fitting you call "city fill". If you leave the bottom connection T'ed into the pump inlet, the pump will suck air with the water. Doesn't matter where on the tank the indicator connections are made so long as they are at top and bottom.
The "city fill" connection should be made after the pump outlet, not into the tank. Otherwise, city water will just run out the vent.
You definitely want a drain valve. I would T it with the bottom level indicator connection at the port you call "city fill"--but it looks very tight there against the wheel well. Can you move the tank an inch to the left?
I've not seen an access hatch such as you show on an RV tank. Many people clean the tank by mixing a cup of bleach with a gallon of water, pouring that in the gravity fill, then filling the tank with water. Pump the solution through all lines and let set at least 4 hours. Drain, fill with fresh water, pump through all lines to flush the bleach solution. Repeat the flush at least once and preferably twice, or until the bleach odor is gone.
I clean the tank once a year while de-winterizing.
Joe
