Originally Posted by M2HB
I realize that's a way to do it. Just not sure if there is a possibility of causing problems with the systems of the MH or the toad.
The short answer is the fuse or circuit breaker (prefered) on each end protect BOTH the MH and toad from any electrical issues.
Assuming the motor home and toad batteries are fully charged when you hook the toad up, you're starting out with very similar voltages on both ends. There's not a real big current flow one way or the other through this wire. While traveling, the draw on the motor home system (keeping the toad battery up to snuff) should normally amount to whatever your brake system is using. 12 guage wire is normally used to do this, but as mentioned, smaller might be OK. There's just not that much required of it in this application.
There's nothing new or tricky going on here. As mentioned, 5th wheel and travel trailers are wired exactly this way.
Starting the toad while hooked up to the motor home shouldn't blow a fuse/breaker. The on board toad battery should have PLENTY of power stored up ready to go for that purpose with a charge line in place. If there's a problem starting for whatever reason, and the toad battery's voltage drops way off from fully charged, THEN you would have a large current flow with the motor home system trying to charge the toad battery - due to the difference in voltage MH vs. toad.