I've always used the onboard converter with an extension cord. Only one RV boiled the batteries and a new converter solved that issue. No problems in many decades of the various RVs we've had.
If water levels constantly need refill it indicates bad battery or converter is out of adjustment. Basically just check the water level every few months to get a feel for how much is lost over time. Never let levels reach the top of the plates and you should be fine for the normal life of the batteries.
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Axis 24.2 "was" tug'n a JK
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