Good feedback above. I had a flat trailer for years and did have an issue with rocks and stuff getting thrown up from the truck's rear wheels. A short barrier at the front of the trailer will stop that. I towed a lot in the winter, so in snow/slush areas the semi trucks going the other way will give the bikes a nice slush mud coat. If you have a newer bike, make sure to put the alarm in "Travel Mode" and return to regular alarm mode for overnight parking. I really like the alarm remote pager. I also request rooms which have a window facing the parking lot (first floor). If that alarm goes off you can expect to see me coming out the window right next to it.
Trailers have been known to disappear from the back of tow vehicles in motel parking lots. The tongue on mine has a lock as well as the bolt thry the trailer hitch. I also have a cable that locks the bike to the trailer and to the truck. It would take a blow torch to cut that cable. I also carry a "weapon of messy destruction" as well...
I finally broke down and got an enclosed trailer. Now the bike(s) stay clean but truck gas when from 22mpg to 12mpg. That gets expensive.
Before you head out for any distance, lift the trailer wheels up and check for loose bearing wobble, inspect the tires, and air them up. Carry a spare tire and a few tools (and a extra set of keys (just don't leave them in your vehicle overnight). Always check all lighting signals because some have a tendancy to stop working (usually corroded grounds)... Stop in the first 25 miles and check your tie down, etc. But most importantly, ENJOY!
jimbob