Another caution when using two [lead-acid] batteries: if they are used in parallel, their must be very closely matched. You cannot use an older battery with a newer one--the new one will very rapidly deteriorate to the condition of the old one.
If you have two new batteries and charge (or discharge) them separately, you have to keep very careful track of the charge and discharge cycles if you intend to parallel them at other times. If they get out of whack, they both head south.
My recommendation is start with two new batteries and keep them in parallel all the time. This sort of defeats a possible benefit of using one at a time, where you get a heads up when you've used half your juice, but I guarantee you will quickly ruin the batteries if you sometimes use them separately and sometimes in parallel. It's just too hard to keep up with them.
I've added a second battery to two of my Airstreams and then a year or two later got rid of the second one and got a Honda 2000i instead. You will be far ahead of the game to have a single battery [that's good for 2-4 days] and a generator, than having two batteries to get you through a week. Plus, you'll be able to run a microwave!
The weight of a second battery is also a problem, especially if you have a longer Airstream. Put the generator in the back of the tow vehicle.