Michael, every once in a while ours quits for whatever reason. Simply "rebooting" it by turning the diesel switch off for a minute and back on seems to reset everything. If you've left yours on for a long time, you might try that. But usually when ours needs a reset, the electric side is out also, so it may not fix whatever is wrong with yours.
As David suggests, even if for some reason you haven't been hearing the burner fire up and run, you can certainly sense it at its exhaust outside if it's operating properly. If not, then whatever was serviced on it last was not done as a particularly complete job, something's gone awry since, or perhaps your mixing valve isn't up to snuff, letting relatively too much cold through; but if it was the valve I'd expect even the electric side alone to be too cold even at the start of a shower.
I assume because you are plugged into park power and your inverter is fine, that your recent battery issues are not involved with the ignition side; still, if you haven't already, I would check the AquaHot's panel in its bay to make sure the LED lamps are all green, and that a low-voltage status from your battery switchout hasn't affected the ignition circuit. "Rebooting" the system with the diesel switch as I described should rectify any faults there, but it won't likely fix a more serious problem with the burner itself or other causative agents.
-Joel
