aux. batteries won't charge with shore power

Your converter is likely where your circuit breaker and DC fuses are are located. Before you replace converter make sure it's actually the converter that's the problem. Check the circuit breaker controlling the converter, make sure the fuses on the converter are Ok, and test the electrical output of the converter at the converter - it's possible the converter is OK and the problem is something like a bad auto reset circuit breaker located near the battery.

If you have to replace the converter check out bestconverter.com - it has a section where a number of people have posted DIY info on how to replace the converter - not expensive nor difficult if your comfortable with electric stuff.