what is it in? I know in the 70's they used to have a switch to change from dc to ac but 90's that should have been long gone.
to be totaly honest it sounds like somthing is not working right, and you probably don't want to hear this, but in all reality you should change out the whole power center to somthing that will be more efficient, better battery charging and filtered DC output. I had a 1991 magnetek in my camper and the first thing I did was change it out with a PD 4000 series power center. not that expensive, I did have to cut the opening a little bigger but thats because I was shopping by price and didnt look for an exact size replacment, which is hard with thoes older power centers.