All good advice above but keep in mind the most common setup seen on travel trailers is 2 24's or 2 27's. It's a decent setup and good bang for the buck. The batteries are relatively cheap and can be had for $80 to $100 each anywhere. Not ideal, but it works fine for 90% of the folks out there.
Once you've had the above, and especially if you dry camp a lot, you start to crave more amp hours. That's where all the excellent advice posted above comes in.
On the other side of the coin are folks that only camp with full hookups and would not consider dry camping. For them, a single 24 or 27 is fine.
My new trailer came with a single 24 with a tray that will accommodate 2, so I added a second battery as we dry camp about 50% of the time. I don't have huge capacity like others but my Honda EU2000 genny is standing by if the batteries get low.
