Hi,
You are not "Stuck" using the same brand, or anything like that. I would not bother repairs either, unless you have something in the $450+ range. Even fixing my $750 inverter/charger was a difficult decision - due to the $450 diagnostic and repair fees.
Best Converter has been around a long time, I would be at least checking prices there. It is really easy to install, only slightly more difficult that hooking up the portable battery charger.
You might even consider installing a solar charging system, and perhaps since you don't really use a 'lot' of 12 VDC power, that will work fine.
SunE;ec.com You can buy a solar panel in the 100 watt range for $100 - $120. Then a 10 - 15 amp controller (good for about 200 rated watts) for a few dollars more. This will allow dry camping for days at a time, especially with your slow electrical use.
My RV has a E-Meter to carefully measure the amperage into and out of the battery bank. I use about 0.8 amps per hour 24/7 to run the CO meter and propane detector. Once the refrigerator is running, then total amperage is 1.5 amps, so figure about 35 AH daily for those loads. This is about what one of my 120 watt solar panels will put out in a day.
Fred.