Engine will only start with AUX Battery Start

Did anyone mention the old standard, DEAD BATTERY?
OK, re-read the OP, he ruled that out.

Disregard.

This maybe of some help Chassis wiring diagram

It seems that a common approach to troubleshooting 12V (and other systems) is to replace the most likely failed part (in this case the battery) without applying some logic. This is often an expensive and frustrating approach. I've learned the hard way to ALWAYS check the stuff first that is free to fix. In this case, clean and check all connections, and using the voltmeter described above, rule out bad cables. If you suspect the battery, don't just blindly replace it, make sure it's full of electrolyte and fully charged, and get it load tested. (that's usually free) Also, check solenoids, which can by bypassed with a jumper cable (be very careful, they can carry a lot of current). Also, check fuses. My C had a 5 amp inline fuse to the charging solenoid that would occasionally blow and the solenoid wouldn't work. My point to this rambling is that with almost any RV system there is a lot of basic, FREE troubleshooting you can do to pinpoint a problem without replacing any parts.

Battery cable connectors and posts on the batteries need to be clean and tight to make a good connection. Ground cables connectors also need to be clean and tight. Take connections apart and use a wire brush and battery terminal cleaner device available from auto supply store. Go buy a cheap digital multi-meter from Harbor Freight, etc. Read the instructions. Set it on the 20 volt DC (direct current) scale. Put the red probe on the positive battery terminal and black one on the negative terminal. With the truck engine running, you should read around 14 volts at the starting battery. Engine off you should read around 13 volts. When your rig is on shore power you should read 13+ volts at you house battery(s), if so, your converter/charger is working. NOTE: The converter/charger does not charge the starting battery on most rigs. Check battery electrolyte levels bi-monthly, add distilled water to just cover the plates inside the batteries using a "turkey baster" type battery filler from the auto supply store. Batteries and connections need periodic maintenance and checking using a voltmeter or they will let you down when you need them. Get familiar with 12 volt DC and 120 volt AC systems in your rig including circuit breakers and fuse locations, it'll come in handy on the road.

I'm heading out there this morning with a trusty voltmeter. Will report back. Thanks for all the suggestions.

No the battery disconnect latching relay does not have it's own battery.
Some have a fuse mounted to it and some have power through a fuse in the battery control center.
Here is link to owner's manual
You should start testing from battery toward the starter relay under the hood. Could be that relay or a bad connection.

I am quite mechanically challenged but OP in his post said that when he turns on the ignition, that there is 'clicking' which I assume is probably a solenoid? Then follows some suggestions that perhaps the circuit might be interrupted via a battery interrupt switch. If there is a battery interrupt switch, it would seem to me that it would be located as close to the battery as possible so that zero current would go down the line to be drained by things like a radio and other power drains. If the battery is effectively disconnected that way, how would power be getting to a solenoid? Do solenoids have their own little dedicated battery?

My ford truck is very hard on my start battery when it sits for a long time. It starts just fine if I am using the truck every three days or so.

I just think there is too much system electronics on the truck that is always on...

During the winter months I get a full charge on the start battery every few weeks when it is sitting more not being used...

Roy Ken

grunt9800 wrote:
Could it be that the chassis battery isn't charged enough?

Yes. What happened the NEXT time you tried to start it ?

It is also possible that you have some phantom drain that shouldn't be there.....like parking lights left on or dome light, etc......in addition to the poor connections already mentioned.

YOU NEED A VOLTMETER.

Some rigs have the battery disconnect switches over the entry door or on the dash.
Ford do use a starter relay and a solenoid on the starter. The aux start would supply additional power to the start relay.