Are Engine and House Batteries Connected?

[Olddud]

On a long trip home, after stopping to get gas, I turned the key to start, and the dashboard went dark. After a bit of panic in the 95 degree weather, I tried the "battery boost" switch, and feeling relieved, it started. I immediately assumed the engine battery was shot. Also, noticed the gas gauge now was pegged all the way to the right and way past "full".

I called ahead to a RV Park and stopped about 2 hours later. Got my voltmeter to check the engine battery, but the door automatic step didn't go down, so I assumed the house battery was now dead. Checked the engine battery with the engine off and hooked to shore power and it showed 6.5 volts, but didn't think about checking the house battery because it was late and I needed to get a new battery before the only auto store in town closed down. BUT, before removing the engine battery, I again tried the "battery boost" feature, and it did NOT start (but dash lit up), so the house battery must have been way down. Got the new engine battery and put it in, and all was well, except the gas gauge was still pegged to the right. Turned the key off and on several times in all directions, checked window operation, locks, etc., and somehow the gas gauge started working right again. Strange.

After thinking about it for a few hours, and worrying I fried the alternator somehow, went outside to check the voltage on the new battery without the engine running, and it was showing 14.5 volts! Checked it with the engine running and it showed 15+ volts.

Questions:
1. Why does, after 2 hours, the new engine battery show 14.5 volts without the engine running -- is it being charged by the house charger?
2. And, are they always connected in some way, or only when the "battery boost" switch is operated?
3. Why did I run down a brand new house battery? Western Kansas is a bad place to almost have a complete shutdown.
4. Have I ruined the new house battery?

By the way, I traveled 400 miles today and everything is working fine, charging right (checked it), etc.

I will check battery voltage on both batteries in the morning to see how they look.

[Oasisbob]

My thought would be a bad relay. I am no expert however. Let us know what you figure out please.

[The_Logans]

On ours, the house battery is charged by shore power, the generator, and the chassis alternator. The engine battery is only charged by the chassis alternator.

This heat can kill a battery REAL fast. Traveling in 100% plus heat we've had to add water every other day!

[enblethen]

Many have a relay that connects the two batteries together to charge the house batteries while chassis engine is running. Many use the same relay as the aux start.
Make and model of MH would help.

[hotbyte]

I'm sure it can vary from mfr, but on ours solenoids connect two together under certain conditions. One allows engine alternator to charge house battery while driving. The other is the emergency boost moment switch if cranking battery is low and needs jumping from house battery. If plugged in and converter is charging house battery, it does not connect/charge cranking battery.

I think I've listed those right...