Dick,
Not sure if I fully understand the relay on the brake light switch.....The diodes are sufficient for eliminating the feedback. There is no need to pull a fuse with the ignition on unless there is an active circuit in the car that drains the battery too quickly like some of the modern Nav systems or climate control. You should not need a relay on the brake light switch if the diodes are in place. It would just mean that your bulbs are getting powered from two sources, both the motorhome harness to the car if active, as well as the car brake light circuit being actived from a Ready Brake or equivalent actuator. The current feedback is still isolated by the diodes. There may be something I have not thought of here but on first pass this seems O.K.....
In the circuit I attached in my post, the relays are not primarily used to isolate, they are just logic setting. They allow the third brake light to be active only when both turn lights are active on the toad. You do not need these relays normally, just specific to the third brake light feature addition. Hope this helps.
Later Ed