I wired a continuous-use solenoid and a relay through a three-position switch. Wire the relay's ground through the switch to a NO oil pressure switch and one to chassis. This way you can make three operating functions for the charge circuit. Switch position 1 is off (never connects the coach battery), position 2 is always on (can be helpful as a jump start if you use a beefy enough cable), and 3 will only activate the solenoid after the engine starts (once it gets oil pressure) The third position is where mine usually stays. Engine running, they're connected. Engine not running, they stay isolated, regardless of key position. A split second after the vehicle starts, I hear a click and I know they're now connected. When I shut the vehicle off, about second later I hear it click off.
I would be cautious about the size of wire you use. The alternator can easily supply a lot of amperage, and depending on the state of charge, the battery might be able to accept a significant amount of amperage. If the battery can take 60A, you'll go through a lot of 40A relays and burn a lot of 10 ga wire, know what I mean?
I wired mine using some 4ga wire and a 100A solenoid so I could use it as a jump start. It doesn't have to be expensive either. I did mine with some old jumper cables that the clamps had become mangled. Solenoid should be about $40-50, relays are cheap.
