No 12 volt or hvac
This was my post on what sounds like a similar issue.
You should look through it for ideas for where it maybe located but this worked for me.
SO long story short ... BEFORE you take it back check these few things out.
UNPLUG the units power cord ! THEN
1) look at your fuse / breaker panel- if it has air vents / louvers then the converter is built in and there should be 2 30-40amp fuses. Check them and even replace them. If they are blown (or if you just replace one or both) plug back in and see if it works.
2) If you do not have air vents louvers then it is most likely behind or under the panel box. (you may have to crawl under the unit and find out where the main cables from the battery enter the unit. You may have to put your hands on the cable and follow it all the way.) Mine was in the closet under a false bottom that just popped right out.
I changed the fuse and everything worked.