Adapting Lithium batteries to old RVs
We have been through this before and two things have become evident.
1. Lithium batteries must be inside a climate controlled area.
Most RVs, if used, encounter freezing conditions sometime in a year. I live in Phoenix. The lowest temperature I saw last year was 12F in my travels.
2. Lithium batteries cannot be directly tied to FLA or AGM batteries.
There will be a constant current drain from lithium to FLA or AGM even if both are fully charged.
If you want lithium find a place inside to store them and provide an appropriate charging system, either a DC to DC charger set to lithium from the alternator or a Lithium battery charger powered by the generator.
With a marine battery switch you can retain the FLA or AGM batteries or you can leave the original battery box empty. Better yet you can use the lithium to charge the FLA or AGM battery.
You can try the drop in lithium but you would be better served to bring it inside.