RickyD, It's mentioned several times, but I don't believe you've answered the question.
Does your water heater have a secondary loop that runs engine coolant through it? I replaced one of our water heaters last year because the fresh water pump, at 45 psi, was pushing water into, in this case, the hydronic heating system, causing the hydronic heating system to "make water", overflowing the hydronic overflow tank. The hydronic heating had a 3 psi radiator cap, the fresh water was 45 psi . . .
In your case I suspect that your engine coolant system operates between, say, 10 to 15 psi, but you water system runs from 45 to 60 psi . . . . so if your water heater has developed a crack in the secondary coils, internal to the tank, the fresh water at 45 + psi will slowly seep into the engine coolant system, even with the engine running, thereby increasing the coolant level in the engine.
If this is indeed happening, the solution is to replace your water heater. Ours was a Dometic, and 22 years old, so it was entitled!