second battery to charge off the engine like the original?

A group 24 or 27 would last for a night or two depending on use. 2 6 volt batteries wired in series would give more.
Look in your owner's manual to see if it has a charge relay for trailer towing. This relay would serve the same as RoyB's after market.
Mounting batteries in the rear you would need to use some prewtty healthy wire to prevent voltage drop no matter which battery(ies) you choose. I would be thinking around a #4. Fuse at 30 amp near your existing battery.