We have a 2021 Coachmen motorhome on a 2021 E450 cutaway chassis. I'm hoping some E-Guru will read this and say "I know exactly what's going on!". If it sits for more than a few weeks, the chassis battery will go dead. As I inquire around, this seems to have become a common issue with current Class C motorhomes on E-450 chassis. Most people get around it by doing a permanent install of a 110 volt Battery Tender to maintain the charge (as long as the RV can be kept plugged in). Earlier coaches had a disconnect switch for both the coach and chassis batteries but for some years now, the chassis battery has been left to it's own devices. I recently installed a blade type cutoff switch on the negative terminal to stop the discharge from happening. I know I can now put a meter across the terminals of the open switch and start pulling fuses one at a time until the voltage reading goes away (This scheme will work as long as it's only ONE "phantom" draw). Before I get into pulling the myriad of fuses, however, what I have noticed is that if the switch is left open for a couple hours, when it is closed again I hear what sounds to be some sort of little motor starting up underneath the air cleaner box then stalling out as though it moved something up against a stop. Would that just maybe be a blower door moving around? As it looks to be a not small job to pull all that stuff off, I'm hoping someone can enlighten me as to exactly what that is and if me unhooking the battery every time we park the rig is not such a good idea. TIA!

 

First thing is do a parasitic draw test to make sure something is depleting the battery, Unhook the ground cable and put a meter between the cable and battery post to see how many miliamps it is drawing. when just sitting. It will be drawing some. Then, you can rule that in or out.

 

Boy , if it was only that easy !

With all the electrical and electronic components in vehicles these days , it is like looking at your TV set and when one little thing is wrong asking a TV repair person exactly what is wrong. Next to impossible to tell without doing some checking and that could require having access to the info, the tools and possess the knowledge to chase down the cause. I hope I put forth a picture that it can be anything , anywhere/.

The noise you hear when reconnecting the power source can be several things also.
Because there could be several components on a particular circuit, pulling fuses is not the best way to go these days. Why? If certain conditions are required to activate the component , if faulty, when the fuse is pulled the system, module , does a reset and when we install the fuse the problem is gone ,,,, until the conditions repeat and the problem again appears.. The voltage drop needs to be measured across the fuses and we go from that info . Certain components will always be consuming power so we need to have a schematic diagram to see what is EXACTLY on that circuit. We don't at this time know if it is something that was installed as the vehicle was upfitted or from original build. What I can share with you is, it will take some detailed testing to pin it down and if the person is lucky,,, may come across it right away or , LONGER.

We do lose some adaptive learning every time we disconnect the battery and it will produce a code.showing the battery was disconnected or power was lost.
Suggest you consider having a good shop or qualified person , with the right info look into this, wish I could provide a better picture, but this is how it appears, right now.

 

After you mentioning it, I realized losing the adaptive learning (and radio presets) is probably why they have deleted the chassis battery disconnect that was common on RVs in the past.