[brucepick]

I'm not sure why you want to disconnect wires. If you're considering switching the charge function on and off, see my very last paragraph below.

Do you not want the red "Battery/Alt" light glowing on your dash? If so, you could remove the cluster and then remove that one bulb. However that will somewhat compromise your ability to run the alternator, if you ever do want to. On the Volvo 240's we drove for 15 years, that indicator bulb and a few others provided the "exciter" current to the alt's small red wire. If enough of those 4-5 bulbs are dead or removed, the alternator doesn't put out any charge.

A related concern -
If your alt belt is removed, I'd say it doesn't matter what else is going on - that alternator isn't going to put out any current. So it's presenting no mechanical load to the engine. In fact that setup will save your more energy than disabling it electrically, because you're also saving the mechanical effort of spinning the alternator, it's bearing friction, belt friction, etc.

You might not detect the mpg improvement due to all the other factors in play - weather, traffic, etc etc etc. But you're taking a load off the engine. You'll likely notice some increase in power available for acceleration or hill climbing.

Definitely charge your deep cycle battery as often as possible. Daily would be nice. The more time it spends fully charged, the longer it will last. That's the basic behavior of a lead-acid battery. And of course charging it off wall current is the way to go, otherwise you're back to buying gasoline to provide a charge to the battery.