I agree with the completely separate truck and trailer lights.......
On my Excursion, I had an issue with my trailer lights for a few months before I even knew.
This truck has the factory tow package, and has the large 7 pin "RV" plug installed on the hitch. I had parked my trailers in the fall (My flatbed to pull the Cubs on, and my Camper) and all was well. Spring rolled around.... (well not exactly spring, it was the middle of March, when we had the camper we would start in mid March, and normally not put it up till between Halloween and Thanksgiving....) and I went and got the camper. I had all the lights working on the truck, but only had stop/turn working on the trailer. It was daylight, so I pulled it to the house and started to investigate. I found the trailer running light fuse blown.... humm.... replaced fuse, turned on lights, POP!!!! So started at the plug and was going to work forward. What I found was, not only did this truck have the factory 7 pin plug, it also had the 4 flat plug. This plug was not in its holder on the hitch, but it was tucked back in along the hitch. It had worn the plastic plug body away over all the years (at this point the truck was 6+ years old) and exposed the conductors. These shorted the next time the running lights were used and melted the plug. I had no idea since the trucks lights were running normally. I removed the bad plug and continue to use the 7 round/4 flat dongle I already had........
Had an 85 E-150 van that was tapped off the chassis harness with one of the Hoppy Tee's.... we ran the 4 round on that van, and the inside of the plug corroded and shorted.... all I had was stop/turn and headlights..... sucked that I worked nights...... had to run both ways with the 4-ways on, till I could get home and get it fixed in the daylight.......
I say if possible, keep them separate......