..Interesting topic here.
I just wired a charge line to our Ford Fusion, doing it the simple way - used charge wire already on back of MH, wired it to toad battery by using one of the wires in the 6-round umbilical cord. Wire was/is already fused on MH side, and I added a 10 amp fuse on Fusion/toad side, right before the battery. Used 10 gauge wire for the charge line as well as ground wire in the toad.
Has worked great, keeps battery in toad charged up fine. Have not installed a diode on the charge line, but now after reading some of this discussion, wondering if maybe I should?
Worst that could happen if you didn't, I'm thinking, is that if you left it all plugged up overnight or longer, you could possibly run down your MH's chassis battery(s), causing MH not to start.
I haven't really worried about that, though, as we almost never do that (leave it plugged up overnight), and if we did and ran down the MH's battery, our MH (and I'm sure many others), has a 'boost' button on the dash you can push that will connect the house batteries to the chassis batteries, to give chassis batteries an extra 'boost' when needed. Long as your house batteries were in good shape (or you were running your genny), you could use the boost button to get things started (right?)
Not crazy about the voltage loss some mentioned, that would go with using a diode. What to do, what to do...