The A frame inserts in to a boss on the tow bar at the back of the RV. It's a square thing, I've not seen one like it over here. It allows you to but a tow hitch, A frame, bike rack, whatever you want really on there.
In an interesting turn of events, I've found a guy in the UK who has in the past towed a Series, a Disco, a P38 and an L322 behind his RV!
He used an A frame and Brake Buddy like I intend to. He said he put the key is position 1 to unlock the steering and power up the cigar lighter (to power the Brake Buddy), and used the other key to lock the doors. Using the key though, not the remote. He also suggested getting a key cut at a locks place and using that. Because it won't have the fob, it would start the car even if someone got in. Sensible!
Lastly he said he just spliced into the wires and there was no problems with the BECM....on my head be it though!
I'm going to go the diode or new loom route, but it's nice to talk to someone who has done it with no problems, so that's nice.