You will not over load your alternator by charging your lithium batteries via your 7 way cable. The tow package has a 30A fuse on that circuit. The most I have measured on the 7 way charging circuit is about 7a.
I installed a Renogy 60A DC-DC charger. I did NOT Disconnect the 7 pin charging wire. I don't always use the DC-DC charger.
A month ago we spent 10 days on Ocracoke Island. We had 3 straight days of rain. No charging from the solar panels. I used the truck and the DC-DC charger twice a day for an hour in the morning and the evening to recharge the batteries on those 3 days. Worked great!!
I think I will get a Honda 2200i companion generator for next time boondocking.
Here's my write up of the install and first time using it.
https://www.montanaowners.com/forums...hlight=charger