On the vent, think easy path for the vapor, every 90 ell adds many feet to the run length. They like 'up'. Heat rises and all that Physics stuff..
I did some research, bottom line is to read the install instructions to see any problems with it. A regular size dryer has a blower that would work with a down draft, not sure about the smaller RV dryers.
If they are routinely doing it, you May be Ok, but read the dryer install book, or call the Mfgr. here is some light reading:
http://ths.gardenweb.com/forums/load...220335007.html