Jayco caravan electrical issues

G'day Nick
My experience is with Projecta chargers in a Jaco but not Jayhub. It is not uncommon for the system to use relays which are a constant drain, depending ON how many items are selected to be ON. Usual for there to be a drain of an Amp or so. The relays simply are the switched supply and ready for the appliance to be switched ON. If your, "GOOD" battery has been in parallel with a DUD one, then I would regard the one you are trying to use as Highly SUSPECT until PROVEN, in charge holding and Amp DELIVERY under some normal loading, ability. It might just be the best of two duds!
Most have a setting, accessible via control panel or APP, to change to LITHIUM battery chemistry and set the appropriate charge rate for it's capacity/BMS to handle.
If normal solar input is not producing much inflow it could mean the battery doesn't accept anything much at all or that unseen resistance is in that circuit, ie On the NEG SIDE most likely!
If those voltage readings are happening it appears the Battery cannot supply and drops voltage with a small load. OR there is a high resistance somewhere in the circuitry which is preventing increasing current flow and therefore less voltage presented to the panel.

I would be removing both batteries and with a jumper leads, substitute a KNOWN GOOD BATTERY instead of the AGM's and see how the system performs, Solar and vehicle charge wise. Just remove the possible fault sources. One CORRODED NEGATIVE terminal can stuff it all up! or positive maybe.

Don't get your hopes up about a JAYCO dealer knowing how it works or how to find a fault.

Also. If you JHUB has a charger which has programmable voltages, you might be able to SET the Charge voltage for absorption to say 14.4v/14.5V and if it gets there is will switch to the lower Float setting which you program into it at around 13.4V. Not sure if yours can do that. Mine is JUST set for the BATTERY TYPE.
Alternately, if Lithium is what you want to use but not catered for by OE unit. or the charge system is faulty, a DC DC unit with solar input could be substituted and connected to Alt feed from car, and Solar feed to it too and the output to the Lithium battery. Alt feed and Solar to the OE unit dispensed with!. Just cheapest way to solve it if charger is not compatible OR FAULTY! Currently running a Kings 25Amp dc dc solar on my ute's lithium batteries. Just a cheapie! Replacing any of these caravan OE chargers is HUNDREDS of MONEY. If their charging features are not functioning, this may be a work around while retaining the UNIT/VAN funtionality!

Nick
The batteries would have been in parallel in the van. If one battery dies as has happened it will usually take the other battery with it. Your second battery sounds faulty as soon as you put loud on it the voltage collapses cause it’s faulty also. New batteries will solve your problem.
Murray

We currently have a caravan with a BMP 35 in for repairs with no solar charge at all. This is an older unit and BMP has told us to send it back for warranty. I assumed it would have be out of warranty. I'ts pre lithium