I don't have a diagram, but I can describe it. It's pretty simple.
I used a 200 amp relay out of a bus air conditioning system to connect the two. It's simply a 4/0 wire running from the house battery disconnect switch to the relay, to the coach battery disconnect. I have a manual toggle switch for now that activates it the relay. One side of the coil goes to ground, the other side goes to the switch and to 24 volts.
Effectively, I'm just adding the coach batteries onto the house batteries in parallel. Same voltage on both banks.
In the future, I will modify the toggle activation switch so it will deactivate the solenoid if either the coach or house battery disconnect switch is turned off. I also want to put a switch in the cockpit for convenience.
Here's photo of the relay when I was installing it. http://www.gumpydog.com/bus/MC9_WIP/Electrical/Battery_Rack/040522.07.crossover_relay.JPG
I only have the house side connected here. I don't think I have one online of the coach side yet.
There are more photos at http://www.gumpydog.com/bus/MC9_WIP/Electrical/Battery_Rack/battery_rack.htm
, near the bottom of the page.