It appears you have access to a volt meter as you checked the battery. Just so you know, the battery at 12.6 Volts is good if the camper is not plugged into shore power (120 VAC). If you plug into shore power and the front A-frame header-mounted battery disconnect is on, the battery should go up to 13.25 or 13.65 VDC from the power converter boosting the voltage. This is not part of the issue right now; just letting you know for the future.
If you were plugged into shore power, the disconnect was on, and you get 12.6 VDC at the battery, the power converter is not working, which will be a problem.
Now to the tank monitor that is not working. This points to a power issue feeding the tank panel. Let's try the more simple part first. If anything I will show is beyond your skill/safety level, please don't do it. I think you should know your abilities.
You don't need to be plugged into shore power; if you are, just unplug it, as you do not need any 120 VAC power for this part of the troubleshooting. You are going to be doing 12 VDC power testing.
Turn the battery disconnect off
for this first test. And make sure the shore power is not plugged in. You want the camper to have no power to it.
Go to the power center to see if the wiring is tight and if 12-volt power is coming out of the fuse clip later. Open the power center door, it will look like this.
At the top and bottom of the fuse block are two small Phillips head screws. They are silver in this pic above, remove them, and the front cover will come off.
It will look like this with the cover-off.
Look at the top blue fuse below the green one, the tank monitor/water pump fuse. Look at the wire on the right side of the fuse and see if it is loose. Check it with a screwdriver. And with all the power off, check all the other fuses, the AC breakers, the white AC neutral wires, and the bare ground wires. It is common that over time of bouncing down the road towing, those screws loosen up, and getting a new to you used camper; it is good to check all those connections to be tight.
If you found the tank monitor/pump fuse wire loose, that is a good find. If it was loose, it might be the issue. So we are going to the next step.
Leave the front power center cover off, get your volt meter and turn on the battery disconnect, and leave the shore power 120 VAC to be off/unplugged.
Look at the picture above. The big heavy red wire at the top of the fuse block is the + 12 VDC from the battery. At the bottom of the fuse block is a big heavy white wire, the - 12 VDC. Check between the big red + 12 VDC wire and the big white - 12 VDC wire for voltage. It should read the same as testing the battery. If not, could you tell us the difference?
Next, test between the wire on the blue fuse for the tank monitor/pump (+ 12 VDC pos) and the big white wire (- 12 VDC neg). You should be getting the battery voltage. If you are not getting battery voltage, that is a problem; report back for the next steps if there is no power at the fuse wire. A problem is something that is more than 0.1 to 0.2 volts lower. If you are 0.01 to 0.09 volts, that is not a big issue; it could be your meter probes etc.
Tank monitor panel measurements
. Assuming you have power coming out of the blue tank monitor fuse, now check the pump switch at the tank monitor and see if the pump runs. If it runs GREAT, you found a problem, and now onto the water heater, see if the red rocker lights up on LP gas mode. Let's talk if the LP gas rocker switch lights up, but the heater does not start to fire off on the gas. That is a different issue than the tank monitor panel. If the gas does start to fire off, shut it down quickly if there is no water in the heater, and you can get back to start-up mode.
Assuming you have found no problem yet and the tank panel switches are still dead, proceed to the below for the next steps. You will only do this if nothing is still not working.,
If the pump does not run, you did not find the problem. First, turn the battery disconnect switch OFF. We will pull the tank monitor panel apart.
Tank monitor cover removal.
These pics are of a two-switch KIB panel, but yours will come apart the same way. Please ensure the battery disconnect switch is off
in case a loose wire is behind the cover so you do not short it and blow a fuse in the removal process.
Remove the front cover with a straight-blade screwdriver, insert, and twist to pop it off. Look for the tab on the side like in the pic to put the screwdriver behind.
The cover will come out like this, and it has a PC board on it for the tank level buttons and LED lights
Sometimes the wires pop off the red connector, watch so this does not happen; you can unplug the red connector from the PC board. Be gentle.
Next, remove the four screws from the white outer ring holding the ring to the wall. This will expose the pump switch; the water heater switches.
See the 4 screws
When you remove the outer ring, wires with wire nuts in the wall cavity also need to come out; gently tug on them to get them out in the open. The fused power wire (12 VDC + Pos) from the power center is in this bundle, and so is a white 12 VDC - (neg)
OK, Now you are hunting for the fused 12 VDC power wire from the power center. From here, you are looking for why that 12 VDC power wire would be dead, with no power. You have a few options pending for your meter skills.
You can do an ohm meter wire continuity test from the fuse block up to the tank panel power wire with the battery power OFF to see if the wire has issues. It should have full continuity.
OR, you turn on the battery disconnect and use the volt meter, and test for 12 volts at the tank panel area between the fused hot wire to the white ground wire at the tank panel area. That 12-volt power wire from the fuse block should be hot/live with the battery on.
If it needs to be clear up what the problem is, please take pictures and post what you are seeing so we can see what you see. Odds or high, the fused power wire lost connection with the entire tank panel. Worst case, something chewed through a wire or something. More once you know more on this.
Feel free to ask if you need help posting pics, and we can help show you how.
I hope this helps, and good luck to you.