The only vehicle I'd recommend thats going to truly meet all your requirements (good fuel mileage, automatic tranny, no alterations) is a Ford Cmax or Fusion hybrid.
I had not mentioned it earlier in this thread because you said you wanted something 3500 lbs or less, and these hybrids are a little heavier than that.
However, some recent posts seems to suggest that you now are thinking outside that 3500 lb restriction. If thats the case, I highly, highly recommend you look into either a Fusion hybrid or Cmax.
'Tis one of the most simple and easy automatics to flat tow - no alterations needed, no fuse pulling or battery disconnecting needed, no crazy towing procedures, none of that, really. Just put it in neutral, hook it up and go. Tow it all day, up to 70mph, no need to stop and run engine every so many hours. And, the fuel mileage is absolutely unbeatable for a vehicle this size. I'm averaging 48-50 wth ours in everyday commuting to work and back.
It will probably weigh a little more than 3500 lbs, though, due to the weight the batteries add. I'm guessing around 3700-3800. Still much lighter than most SUVs and other vehicles you were probably considering.
Anyway, just one other thing to think about, consider.