If the electrical system is the same as our 96 Hymer, it's a constant voltage charge setup, with a 10amp max. So when you hook up it will take your batteries up to 13.8 volts and then just trickle a few milliamps into them to keep them at that level. Ours didn't actually charge the vehicle battery as standard, just the leisure battery, which i thought was a bit poor. If you look on the control panel at the battery levels for battery 1 & 2, it should show just under 14 volts while on charge, after being on charge for a while. If battery 2 shows a lower charge, and stays around 12.5 ish, the vehicle battery is probably not being charged.
So assuming Hymer haven't changed, it won't do them any harm, and won't cook your batteries.
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