Isolating TT's Lithium from TV's Lead Acid Batteries

Do you really have to isolate the Lis from the TV batteries? Why? Going down the road, you might be getting 13.9 volts from the TV and 14.x from solar, so in that case the solar will do the charging.

If no sunshine, who cares if the Lis are charged at 13.9 volts for a while? When you get there, you unplug the 7-pin and now you are isolated anyway.

I can see it if the 7-pin were charging at a higher voltage than the Li spec calls for, but who cares if they are getting less voltage than they can handle?