I don't see an issue with a butt/crimp type connector on a #8 or #6 wire along with heat shrink tubing. Soldering the wires would be another option with heat shrink tubing, or along with a wire nut depending on where the splice would be.
A properly done butt connector & heat shrink tubing is going to be a way better than what the factory does in A-frame areas in a TT where crimp connections are used and are exposed to water, dirt and salt. The photos below show the two mini-breakers in our TT. I recently moved all the connections and 2 breakers into a watertight box. The crimp on the ring terminal on the + wire from the battery was barely crimped and when I touched it, the wire fell right out of the terminal. No heat shrink tubing used on these terminals either. This was potentially a dangerous situation that could have resulted in loss of the brakes. Heck, I even had a few wire nuts upside down in the 4x4 metal box with water inside them. All done to code at the factory... :R

