I have a 2015 and thought I had the same problem. However, the battery only charges on my trailer when the truck is in gear. Took me awhile to figure this out. I had to test it through the trailer wiring with my wife in the truck with her foot on the brake and the truck running and in drive. The battery stays charged just fine. I ran the interior lights off of marker lights. So, the lights in the trailer work when the parking lights on my truck are on. You could also run them off an auxiliary battery, but if you forget to turn them off……My trailer isn’t a travel trailer. Just an 8.5x20 enclosed trailer with LED interior dome lights.

 

I solved it with a 12V DC/DC Charger

File attached

 

I have also installed the Renogy DC-DC Charger. I put mine on the camper with a trigger wire to the parking lights. As long as they are on, the house battery is getting a charge. When the truck is shut off and the lights are off, there is no worry about draining truck battery

 

I will have a 2nd camper in a year, so put it on the truck, so I can use it on multiple campers.