Now this is a thought, could the inverter in-fact be wired directly to the ac panel then go to the relay? It is tight behind the panel and I did not open the rear cover to the ac panel to trace those wires that lead from the relay to the panel. Guess I need to investigate.
I just visited the Xantrex website and looked into their discontinued inverters. All they had was a data sheet for the PW3000, but they did have the installation guide for the PW1500. It appears neither comes with an automatic transfer switch and has receptacles on the face of the inverter. Before you start messing around, BEFORE, I urge you to learn all you can about this unit, even Google finding a manual. If YOU don't understand how the thing is wired, you should learn. If YOU don't understand that as long as the unit has DC power CONNECTED to it, it is live, REGARDLESS of what the remote switch position is, you should learn, 'cuz that's what the bloody manual says.
IIWMB, I'd draw a wiring diagram, first.
I also would hesitate to use a relay for the important transfer to assure that two A.C. sources never feed the circuits. There are reliable mechanical switches for this purpose.
If that relay fails, and you get two sources of power at the same time, you will be SOL.
Good luck, just trying to warn you of the dangers.