If I understand the problem correctly the potential for conflict is not one that will harm the electrical systems if wired properly (presumably with diodes as you have stated). It seems the potential for conflict comes in on some installations when you put your turn signal on in the motorhome and the toad turn signal starts to flash with it. Good so far. But, as you brake in the motorhome with the turn signal on, if the auxiliary braking system also applies the brake pedal in the toad, the power from the toads electrical system will illuminate the toads brake lights and in some cases where the brake light bulb is the same as the turn signal bulb the toad tail light will appear as a solid non-blinking light since it is getting continuous power from the toads battery. The result.....the motorhome's blinker keeps giving a blinking signal but the toad seems to stop blinking and illuminates the brake lights.
There is a modification for this if it happens in your installation. We were concerned about it at first but so far have done nothing about changing our system. Our theory is that since the auxiliary brake is only applying itself for a few seconds and since the motorhome's blinker keeps working throughout the process, there really isn't much of a problem....if any. Cars behind should still see your intention to turn and the toad, while momentarily not blinking, will show a brake light.
This may or may not happen on your installation. You' ll have to check it out. But if it does and if you decide to do something about it the etrailer relay product may be your answer. You have to make the final decision as to what is safest for you and gives you peace of mind. Good luck and safe travels.