You wrote: <I have AGM in the truck and Li in the camper.>
1) AGM and LiFePO4 batteries have different chemistries and different charging needs.
2) The AGM battery wants about 13.8 v to 14.4 v during bulk/absorption charging phases dropping to about 13.5 v for float (maintenance charging)
3) Lead acid batteries will degrade if chronically overcharged or undercharged so always fully recharge before discharging again as well as avoid routinely discharging below about 50% SOC (50 % of full charge).
https://batteryuniversity.com/article/bu-403-charging-lead-acid
4) The LiFePO4 battery wants 14.4 to 14.6 Volts during bulk charging
5) The LiFePO4 battery wants around 13.5 V float voltage (many adjust this lower - see below)
6) However, Li-ion does not need to be fully charged as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge because a high voltage stresses the battery. Choosing a lower voltage threshold or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime.
7) A lithium battery charger will damage a lead acid battery by overcharging it with high voltage
https://batteryuniversity.com/article/bu-409-charging-lithium-ion
All that written the key may be where the ACR is installed and perhaps the size of the wires (if they are too small min of 6 ga is recommended) It should be in the truck close to the AGM starting battery. Also that assumes your truck does not have a smart charger.
It seems that the ACR is not protecting the AGM battery for some reason (not switching off) and what is happening is that your charge controller is trying to bring the whole system up to the voltage set for charging the Lithium battery bank (is your charger programmable?). If that is true you are constantly overcharging your truck battery. Another possibility is that there is something in the truck constantly putting a small load on your truck battery (an accessory or bad connection or something).
But I am confused because you seem to imply by your comments that you also have a DC to DC charger. If that is true I am pretty sure you do not need an ACR. It would be helpful if you could describe the system a little better. A diagram would help.
By the way both ends of the wire between the truck battery and the camper battery (which I recommend should be at least 6ga) should have a large fuse 120 Amps or more. Blue Sea resettable fuses are reasonable.
I do not have hands on experience with DC - DC chargers and smart alternators. Ntsqd or Vic or Rando, if he is still around, may have better insight so hopefully one or more of them will weigh in.
I hope this is helpful and not too confusing
Craig