MH wont start on chassis battery. Has 13V

An engine starter high current circuit should not contain more than one power relay.

A test should be conducted involving the following protocol...

Have a second person available to stand outside and watch the headlights.

NOT utilizing the battery parallel switch try to start the engine

Do the headlights dim or go out?

Is this a Ford V-10. We had the same problem and it just got worse to the point that it wouldn't start even with the boost, and we had a new battery and starter in it. Turns out there is a 2nd started relay that is not shown in the diagrams with difficult access from the dog house or passenger side fender area under the hood. DH got it changed and no further problem. He had read about it on a forum and then did research about it to find where it was located.

Ummmmm interesting stuff.

The alternator can maintain the starting battery at peak voltage and run all the accessories of the chassis because it develops it's own negative.

But if the main chassis negative cable develops a high resistance due to battery post to cable or cable to frame corrosion or looseness, then a a certain amperage this cable will fail to complete a circuit.

That cable, the chassis battery cable, has nothing at all to due with the house batteries. Either positive or negative sides. When the paralleling switch is enabled the house batteries do all the work. The chassis battery is leaning on a shovel.

It's time to clean and tighten all four connections of the chassis battery. Paying particular attention to the cable to frame (or engine) negative connections.

Try this and see if it helps. I have seen far too many bad cables where the copper stranding inside the cable fails where the stranding meets the terminal. So do not rule that out if cleaning connections alone does not help.

In case someone runs into this problem (healthy chassis battery that will not start up the coach, but it will start with battery/boost), here is the problem. Hopefully if someone does a google search they will find this helpful.

I called two mobile RV mechanics and they could not find it. One blamed the batteries ("there must be a bad cell") and the other said "everything is working fine. we have 14V at the alternator and 13V at the batteries -- 1V drop is normal for that big run". LOL. When I told him we had 13V before we turned on the engine ... so it didnt go up at all he just looked stumped. SO much for being an ASE certified mechanic (and he said he was the chassis specialist)

The problem is very easy -- there is a 600A fuse in the electrical bay. It is is in the way back and impossible to miss. It was blown. ohms reading didnt help determine it was failed (cant explain this -- but when removed ohms indicated it had failed), So I took voltage reading on both sides and found one side didnt have voltage. Once replaced everything worked again. I shutdown power each time I went back there because of all the high voltage in the bay (steps: shore power off, tripped the echo charger breakers, turned off both inverters, disconnected chassis batteries, turned off solar arrays, and then hit the disconnect switch in the coach).

If I had to do it again (and I hope I dont, but if so, not a big deal now), I would have the replacement in hand first so that you can do the swap once and not repeat the disconnect steps because of the poor location of the fuse.

You are measuring with a hand held meter and not relying on a dashboard read out?

the converter charges the HOUSE batteries not the Engine batteries on most RVs

on some Rvs there is voltage controlled relay combiner called a 'smart relay' that will connect the chassis/engine batteries, after the house batteries reach a set voltage

your batteries are not being charged by the alternator
you have a problem somewhere
in the wiring, turn on the head lights, take a battery reading
then turn on the dash a/c and take another reading
if i had to guess,
at this point i would say one of the positive cables did not get reconnected
you have dash power but no 'starter' power until you push the jumper button

Do you have a combined charger/inverter? What is the house battery voltage on shore power?

The alternator should keep the chassis battery at 13.4V+. So the batteries aren't being charged and won't start the engine.

Some rigs charge the chassis battery from shore power and some don't depending upon configuration.

I purchased new battery post adapters to replace the old ones today. Cleaned all wiring up. Still the same problem!

While doing this I turned off the chassis battery. The dash is completely dead. SO obviously it is getting some power when the batteries are hooked up and on.

I fired up the coach with the battery boost and then took a reading on the batteries. 12.70Volts. If the engine is running, would you expect to see 12.7V or should there be a higher reading because they are being charged?

Also.. when the coach is hooked up to shore power, wouldnt you also expect a higher reading (something like 13.6) because there is no load on them and they are presumably being charged? (or maybe they are in taper/float mode so it wouldnt be so high?)

I am not sure what to do here.. the batteries were working fine and now they are not cranking the engine. They also dont seem to be getting a charge? But they are not completely disconnected because the dash sees them.

I am trying to find the battery charger and I cannot find it on the coach. I thought country coach had a dedicated charger for the bank. But could I be getting 12.7V and the batteries are too weak to crank the engine (nothing from the starter)?

Some coaches have disconnects for both house and chassis
My uncle's Thor Serrano is that way