I bought the 709-PH model. It measures the fresh, gray, and black tank levels, plus the LPG and a digital voltmeter. It has a switch for the water pump and the hot water heater. I never used the hot water heater switch, as the existing one is on the other side of the coach.
SeeLeveL Gauges by Garnet Instruments Ltd. - Model 709-PH
The links on the web page will allow you to look at the installation and shop manuals for the unit. They explain how it works.
IIRC you need to specify that you need a 3-way switch for the water pump unless you have installed a water pump relay, which I never did.
The 709 panel is smaller than the existing panel. I cut a 1/4" thick wood panel the size of the existing tank gauge to cover the hole and then cut a hole in it to accommodate the 709 panel.
My gray water tank is L shaped and was taller than the 12" limit for one sensor. I bought a second one to combine the two as described in the manual. If I was doing it again I could consider just using one 12" sensor, and know I had a little safety margin when the tank says it is full. The 12" sensors are fine for the black tank and the fresh water tanks.