I installed a Xantrex Heart Echo charger (
Echo Charger) this past weekend after my Trik-L-Start failed. I don't think it's bi-directional like the BIRD. It keeps the chassis battery charged when the house batteries are being charged, but not vise versa.
My Allegro also has the "assist" button you speak of. It's just a heavy duty solenoid (around $20:
Solenoid) that connects the 2 battery banks together via a momentary switch in the dash. It is meant to provide emergency power to the chassis so you can start the engine with a dead chassis battery. It's very simple to wire up.