You didn't say how old the engine is. Older engines only had a rectifier and no regulator. Basically used the battery to regulate the voltage, boils them a bit but the idea was, you could always top them up. Seeing 18 volts (floating) on this would be normal
This setup shouldn't be used with sealed leisure batteries as it will quickly destroy them.
I think it was about 1990 (I could be wrong) that everyone started fitting regulators to their charging systems to facilitate the use of leisure/gel batteries. Should show about 14.3 volts "on load".