Converter putting out 16.5V, some appliances don't work.

Very similar to pulling the regulator out of the LPG system and saying "Oooooooo look at the stove and hot water heater, now"

Your system is wired all wrong for circuit isolation. If when plugged in your converter delivers more than 13.5 volts for floating the batteries, it is a battery killer. Old converters are hammer and chisel guesswork on the part of the manufacturer. I would be worrying about solar compatibility, down the road.