More info - it looks as though the Roadmaster Relay Kit only works with Roadmaster braking systems - or at least one of the wires is supposed to attach to the braking system, so I guess that's out.
It looks like my only option is to use a 2-wire to 3-wire converter like the Tow Ready taillight converter found here: Electrical Wiring - Tow Ready
, and then add 2 more diodes and all should be ok.
I realize that both the RV and the Car will be trying to light the brake lights using this setup. I'm still curious if there's a way around this. At the very least I imagine I'll need to charge the cars battery while towing to prevent it from running down.
Any people towing a newer Honda Fit? How did you resolve this?