Hmm made me think of questions more than answers-
The ground wire should be at least wire size of the charge line. You didnt mention if you upgraded charge wire size.
By tailight ground I assume using the ground within the trailer/camper plug?
IF the solenoid you added breaks the hot charge line (didnt say if you added new charge wire or are interupting existing line) your good.
If existing or prior line still connected and not controlled by 'solenoid' then, depends on whether or not your truck kills power when off? Assuming your truck doesnt or added solenoid to get more charge?.