Battery Isolator Location and Type

I would look under the hood near the chassis battery. Many Ford chassis MHs use a silver can relay to charge the coach battery. It energizes from a hot in run position.
You will find a starter relay that looks the same.
The other possibility is a diode based isolator. Many of them are 4X6 inches blue in color also located under the hood.

Suspect you have something similar to this - often used on older Ford Chassis rigs. Likely located in the engine compartment.

If your goal is to charge the "house" battery while driving and not let it drain or discharge back into the starting battery, you should read this.

DC-DC Battery Charging

Not cheap, but it will solve your problem.

Thanks for the suggestions. After looking everywhere I could think in the engine compartment, I traced what I assumed was the cable going to the isolator, it was connected to a black box marked Hi-Amp Buss 100amp, I assume that is a fuse but anyway the cable disappeared down in to the engine compartment an out the bottom and back about half way, passed the coach battery, then up through the floor.

Under the back couch and behind a plastic shield I found it. With the engine off I measured two different battery voltages on each side and nothing at the actuator connection. I started the engine, I then had voltage at the actuator contact and guess what, it was now working showing alternator charge voltage on both sides. The meter that I had been monitoring the coach battery agreed. That meter has never shown above 12.5 volts when the B wasn't plugged in to AC. Needless to say I will be keeping an eye on it I think something is intermittent.

Here are a couple of pictures.


100amp Buss


Hidden pieces.

I would clean and coat the connections with electrical grease on the Buss Hi-amp circuit breaker. They look a little cruddy!
I know it is toom late for now, but get make and model of the devices in the "hidden" box. Maybe take a photo for future reference.
Glad you found your problem or at least know where to look if it comes back!

theoldwizard1 wrote:
If your goal is to charge the "house" battery while driving and not let it drain or discharge back into the starting battery, you should read this.

DC-DC Battery Charging

Not cheap, but it will solve your problem.

Looks interesting. I'll keep a bookmark to it for future consideration. I'm still sorting out this B to understand what is what. I will probably need to swap out the charger/converter at some point. I plan to add a second coach battery, still debating if I should upgrade to something like Lifeline batteries.

enblethen wrote:
I would clean and coat the connections with electrical grease on the Buss Hi-amp circuit breaker. They look a little cruddy!

Good catch I will clean them up. I was so busy looking for the isolator I didn't even notice the corrosion. That could explain the difference in voltage between the dash cig lighter and the coach outlet when the engine is running. Well that and the 15' of cable that could probably be bigger.

I got the Buss Hi-Amp cleaned up today and also discovered the battery cables on isolator were just finger tight, if that, so tighten them up, that may have been the intermittent problem.

While digging around under the couch again I'm now thinking maybe I should switch to AGM batteries and install the second one under the seat closer to the all the action. I assume I would have to switch chargers and maybe buy one of those expensive DC to DC boxes to charge them, or is there a different way to control the charging of AGM batteries?