Considering the test with the 05 and 16 model trucks I mentioned above I think 8.5v is just all this truck and controller are going to give. Ill have it all hooked back up here ina bit and as long as the brakes are strong ill be happy. Im honestly tires of messing with it, I have almost $2,000 in the new wiring and new brakes and paying a friend to help. I only paid $5,500 for the trailerDepending on how your controller is set, it may not be pushing full voltage when the trailer is connected. With no load, the controller pushes full voltage, but there is nothing there to absorb it, so it stays up.
Check your truck side plug for proper tension and corrosion on the lugs. They may look clean, but actually be slightly covered in corrosion (like aluminum powder)