I've got a class A motorhome. Yesterday had a really bad rotten egg smell after driving - tracked it down to the engine battery. It was hot and showed signs of overheating (sides has swelled). Replaced the battery today and thought that would be the end of it. But guy at auto parts store said a battery only goes dry from overcharging - so your alternator/regulator could be bad.
I know that battery had to be going bad for at least a couple of weeks. The last trip out (two weeks ago) I had a dead battery - got it started using the emergency start switch that uses the coach battery. I know I should have addressed the problem then.
Bad news - I am six hours (near Ithaca NY) from home. At least an hour drive to the closest RV shop that can see me in the next day or two.
The question - could driving six hours cause the water in the battery to boil off? Or is this something that would take a long time to occur. It could have just been the battery going bad and nothing is actually wrong with the charging system.
Unless the voltage is pegging outside normal ranges, 6 hours wouldn't boil a fresh battery dry.
Starting batteries, like deep cycles, need to have the electrolyte topped off with Distilled water periodically as they will lose water from regular use.
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I would be concerned about polarity.
You should verify that the house and chassis batteries are connected correctly. If they are not, when the charge relay or what your rig has energizes it puts a direct short across the system