I was in the same boat when I started. The panel I can't really help with and I've never seen a manual. I can tell you they're not very accurate anyway so only use the indicator as a really general gauge. For instance, the 1/3 sensor is usually only about an inch off the bottom of the tank..think of it as a "not empty" light instead of 1/3. May be as simple as removing the panel and checking for broken wires.
Your other question depends on what type of regulator you have. Post a pic if you can. Yours kind of sounds like my old one. It was manually switched. Point it towards the tank you want to use. It would operate in the middle and pull from both tanks, but you might get a surprise and run out of gas with no warning. Using one at a time will at least let you know you're empty on one tank and need to refill. The other kind is auto switching and both tanks stay open. Those usually have some kind of "empty" indicator and read whichever tank the switch is pointed towards. That tank is the primary tank. On mine, when the primary tank runs out, it switches to the other tank and the indicator turns from green to red. Pop the empty tank off when you get home and refill, put it back on and turn the switch to the other tank (which then becomes the primary tank). Repeat. Again, it depends on the type of regulator.
2012 Chevy Tahoe LTZ - HD tow package
2017 Rockwood 2703WS Emerald Edition