When the power is connected to 20amp service the 110v circuits work but the 12v lights/refrigerator are very dim.  Could it be the converter is bad?  The RV is a1992 Prowler.  Everything worked fine 3 weeks ago.

Is the converter charging? A quick test is that you should notice the 12v lights brighten when you plug it in. You can also measure the battery voltages before and after plugging in. The voltage should increase after plugging in. If not, look for a possible converter fuse (if there is one it should be with the other 12v fuses on your 12v /110v distribution panel/converter unit).  The fuse might also be an in-line type in the converter feed wiring. If fuse is OK (or don't have one) check for loose wires (especially a ground) for the converter. Follow the + & - wires (especially the - wire) from the converter to their sources and make sure all is tight. Ground issues are the Rvers bane. Otherwise you might be looking at a converter replacement which is really a fairly simple deal.

Also, you may want to check the battery, cables, and water in the battery. The converter will not work right if there is a problem with the battery.