So, I've further worked out my plans for the teardrop battery. How does this sound?
Have a 10 gauge charging cable run to the trunk of my tow vehicle. in the trunk, have this switch mounted: http://www.delcity.net/store/Battery-Disconnect-Selector-Switch:-BOTH,-1,-2,-OFF/p_791777.a_1
On the input side of the switch, wire the battery to a powerpole disconnect.
On the "battery 1" side, wire to the car alternator.
On the "battery 2" side, wire to the trailer.
Then I could disconnect it (when the car is stopped), connect it to the car to charge, or flip to the TD for power.
The battery will be on a powerpole connector, with a matching one on the trailer and on the car receiver, so that I could take the battery out and carry it to the trailer if the car wouldn't pull up to it, or connect it up through the car. Are the powerpole connectors safe for use outside?
Is there a reasonable option for having the battery charging cable turn on and off with the car engine? Something like one of these? http://www.delcity.net/store/Solenoids/p_790159.a_1
Can anyone recommend a fuse holder for 10 gauge to mount with the battery - maybe this? http://www.delcity.net/store/Maxi%C2%AE-Fuse-Holder/p_11183.a_1
Is all of that clear as mud?