I would not recommend this. There should also be a diode and fuse between the coach and toad. The object would be to allow current only from the coach to the toad (diode). The fuse for overload problems. In addition to all this, one would need to add a device so when the coach alternator is not charging the coach battery, there would be no current flowing to the toad. It would be a bad situation to have the coach battery drained because of the toad.
I took the easy way out. Go to http://www.lslproducts.com/ToadChargePage.html
and consider this product. It is what I use.