Toggle switch would work to close the coil from the backup battery, just make sure that this switch doesn't itself parallel the two batteries as it would fry pretty quickly. It needs to be a SPDT to select it's power source from either the fuse box (your "truck running" source from Battery A) to only battery B. Remember, this would leave them tied together in that position until you turned the switch off.
As for the batt ground, if two cables don't fit, you can just upsize the factory ground lead and tie the to grounds together on the battery side of the sensor. May even be easier to practically implement.
But you're missing something here with tying two batteries together all of the time. If they're always tied together with cables, you can't run into the scenario where one is very depleted, and the other is charged. In your scenario with the relay, you could have one battery at 10V and another at 13.8V, and THEN you would be tying them together. That means LOOOOOOTS of amps!!! Battery isolators also prevent this scenario. As for others who have done this or done that with all kinds of success and no problems, maybe they've been lucky is all I gotta say. Russian Roulette is tons of fun....until it isn't.
It's the lot of neglect to one battery with your setup.
And don't take my comments as arguing or anything, just trying to help I have just a bit of experience with cars, batteries, electrical systems in general.