I think I would try to isolate where the leak is by disconnecting the lines to the hot water heater. If you are still using coolant then it's got to be engine. You said that there was no pressure in the overflow with the cap off while running but wouldn't it be more apparent if you left the cap on so the pressure built up? There is a way to check if exhaust gases are present in the coolant, an automotive parts store should have it.
SV Laurie Anne
1988 Brewer 40 Pilothouse