Do battery isolators damage your deep cycle coach batterys?

Diode based isolators are not a big problem in RV's since they are NOT the prime charging source for the house batteries. They are quite capable of maintaining a good level of charge on those batteries from the alternator while driving. Unless the rig never sees shore power, to allow the converter/charger to fully charge the batteries, there will be no lasting ill effects.

The average motorhome spends many more hours on shore power than it does going down the road. The house batteries will see a full charge often enough.