My neighbor told me that many existing lead-acid chargers will charge lithium batteries as long as you can set the max. voltage of the charger and as long as the charger does not have an automatic equalization mode.

Okay, one caveat to that is most modern vehicles have a "smart" alternator. When we did the conversion to LiFePO4 battery, I found that the tow vehicle was getting power from the trailer. I started a trip with a full battery and the alternator just loafed because the LiFePO4 battery's voltage was high enough that the alternator's programming "thought" it i did not need to provide power. I ended up on that trip with a rather depleted LiFePO4 battery. I then realized that I needed to do something different so I added a DC to DC converter from Renogy. To power it, I added a dedicated circuit from the tow vehicle. I could possibly have used the 7-way connector and just disconnected the wire underneath the trailer and run a separate wire to the DC to DC charger, but I did it this way so that I could also put in a battery isolation solenoid. This is the one I bought:

https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/ - https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/

I do have a write-up in my mods detailing that conversion. With the Grey Wolf which has solar, I got an MPPT controller that can also use the solar as well as the vehicle for input. I still have the DC to DC converter. I would be willing to part with it if you would like to put it to use.