Explaining what you need is fairly simple.
On coach side, you typically can use a 100 amp disconnect. Please note that inverters are typically connected directly to the battery not through the disconnect relay.
On the chassis side that you need a bigger one in order to handle the starter amperage. This 200 amp is for a gas rig. A diesel may need a larger one. The rating (200 amp) is for continous ON. They have a instantanious rating of 400 amps which supports the high starter current.
The disconnect relays do not require a constant -12VDC signal to the them to change position. A pulse signal is sent to them that causes them to shift position and then mechanically latch in place (no parasitic load that way).
For add-on, they would be controlled by control panels simular to these:
Most of these items can typically found cheaper on EBAY (search INTELLITEC). It is the labor to install and run the wiring for all this that is the cost (someting like $150/hr?).
) is just one source. There are others.