If your 2003 is wired as my 2008 was, the following applies. If not, well...
The converter connects to the house battery bank through a bus bar. The bus bar is attached to the (rotary, manual) house battery bank disconnect switch.
The house 12V service passes through the same switch. Then, it goes through a 120A breaker. From there it goes forward to the Front Distribution Panel (FDP), which is located in the forward-most driver-side storage bay.
On the FDP, the house 12V service splits. Part goes to a bank of fuses. The rest goes to a solenoid. This is the house battery disconnect solenoid. From there it goes to more fuses and through a 40A circuit breaker, on its way to the rear interior 12V fuse panel.
The BATTERY or BATTERY CUTOFF switch activates the house battery disconnect solenoid. Often this switch is by the entry door.
If you read through what I've written carefully, you'll see it is possible for the converter to be charging the house battery bank (say, 13.5 volts) even though there is no 12V to the interior of the coach.
Check these when you notice there is no 12V to the house loads:
1) Is the LED lit on the propane detector?
2) Does the display on the refrigerator work? (This assumes a propane-electric absorption refrigerator)
If the answer to both is "No," then check the 120A breaker near the house battery bank. This is because 12V is not getting to the FDP.
If the answer to both is "Yes," then check the BATTERY or BATTERY CUTOFF switch and its solenoid. This is because 12V is getting to the FDP, and to the "unswitched" loads such as the propane detector and refrigerator, but is not getting though the house battery disconnect solenoid and/or the 40A breaker.
I suspect the house battery disconnect solenoid. I had to replace it once on my coach in six-and-a-half years of ownership.
By the way, I don't think the original converter would have been a Powermax. More likely is a Progressive Dynamics PD9100 or PD9200. Link to them here