Wish I was in on that trip!  But, we did go down that way in June.  The Sea Dell Motel is a great place to stay in Marathon.  Parking is kind of cramped if you have a trailer, 

 but they are good to work with you.  The place is very clean and reasonable considering.  If they aren't real busy, they will work with you on the rates a bit if you try.
If you are riding around up 'north' in FL, the Clay County Jail Museum is great. 

  I will try to get some pix up some day.  The jail won't take too much time, but it was a lot of fun & a good break.  My only time ever behind bars. 
  
Oh yeah, a couple of great places to eat is the Keys Fishery in Marathon, It is on the Gulf side, I think at 36th street.  The other great place is the Square Grouper, it's at the 22.5 mile marker (I think), I don't have that shirt on. :s  I am wearing the souvenir from the No Name Pub, on of course, No Name Key, off Big Pine Key.  Watch out for the Key deer, tiny little buggers.  
Have a great trip, we've spent quite a bit of time in the Keys, PM me if you have any other questions. I'll help if I can.