You don't need to worry about stressing the alternator in your tow vehicle by charging a lithium battery over the 7 pin connector.

Essentially the wire gauge is so small and wire run is so long that the voltage drop doesn't allow much current to flow anyway. 

Here is a video from Battleborn briefly going over this topic: https://youtu.be/Gxypk8J-0Mg

Generally this is only in issue in Class A, B, & C drive-able RVs where the starter battery & cabin battery exist in the same electrical system. 

Travel trailers, 5th wheels, & trucker campers do not have this problem because the starter battery & cabin battery are in separate electrical systems and the only connection is through the 7-pin.