I put a switch on mine as well. Not so much to have the coach battery boost the engine battery (although the switch would work just as well as a push button switch if the need arises - but don't use it to get more TV time if your house battery is dead!) but to be able to keep the batteries isolated rather than have them connected any time the ignition is on. At least on my '91 Ford chassis, the push button "emergency start" relay would engage anytime you turned the key - in the 'on' and 'start' position.
The main reason is I'm usually boondocking; so the last thing I want to do is leave early in the morning with a heavily drained house battery and as soon as I start to crank the engine I'm connecting the lower voltage house battery to the chassis battery which is now sending a bunch of amps to the house battery instead of going to the starter.
Then after the engine starts I'm forcing the alternator to deliver 40+ amps to the house battery in addition to the amount going to the chassis battery to top it up after the start.
I would much rather keep the batteries isolated, let the chassis battery crank the starter, and let my solar charge the house battery while I'm on the road.
But if I want to give some solar to the chassis battery, or give some alternator amps to the house battery, I can easily do that with the flip of the switch. I used a lighted toggle switch so that it is obvious when it's engaged.
