I am not sure if you have done the relay mod for the front headlights that brightened them, I think you probably have. I suspect that the wire length required to get power to the brake lights and tail lights may have an issue with voltage drop also. If you apply the brake lights with the bulbs in place by piercing the wire covering see what the voltage is at the rear tail and brake lights. If it is not over 12V you could apply the same wiring change adding a relay to those circuits by adding fused wire leads from the battery compartment and using the current tail light feed wires as a triggers on the relays.
There may be a good alternative LED assembly given they are not as voltage sensitive as a filament bulb regarding brightness. I would try a couple of LEDs and see if you find one you are happy with. If not then check the voltage near the bub base and decide if a modification is in order. This is especially critical given the load of the car in tow is yet another added load on the coach long wiring harness adding voltage drop. You may want to check the voltages with the toad wiring and lights attached to see what the real world voltage is. If you do a mod with relays I would probably use 12GA stranded wire from the battery compartment to feed the relay switched lead and mount the relays near the tail lights. Hope this helps.