You are right about the diode not preventing the motorhome chassis from being drained by the toad while parked overnight.
The SMI Stay In Play has only drained the toad battery twice and it happenned after two days of towing and leaving the SMI "ON" while parked overnight. I guess the battery was very low after two days of not running the engine and leaving the SMI switch ON overnight was the drop that broke the camel back.
While I want to be able to move at a moment of notice while staying overnight at a truck stops or a Walmart, I don't want to face dead chassis batteries and not being to move out quickly. I am considering providing 12 volts to the toad using the coach batteries (4 golf cart batteries). I don't think that I will ever face dead coach batteries unless something is drastically wrong.
2006 Dutch Star 4023,
TST TPMS, SMI Stay-IN-Play, Blue Ox Aventa LX,
2011 Toyota 4Runner w Drive Shaft Coupling