You didn't indicate when you are planning the trip but you can use this site to check road conditions in CO.
http://www.dot.state.co.us/
Wherever you decide to stay in Moab or surrounds, it would probably be best to make reservations. Moab gets pretty busy in the Summer
This is a Passport America CG in Moab however I have not stayed there so I cannot give any info about it.
http://www.okrvpark.com/
Four days is just about right for Canyonlands & Arches. This would also give you time to drive South on 191 to see Hole in The Rock, Wilson Arch and Newspaper rock (North of Monticello). There is a shortcut to Monticello from the road into Newspaper rock. If there are signs indicating it is closed, believe them (personal, stupid experience).
You will also have time to wander around Moab and as far as eats, Moab Diner and Pasta Jay's. Both excellent places to eat and I am sure there are others as well.
It you think about taking hwy 128 into Moab from I-70, be aware that it is quite twisty and there are overhangs on the hwy. With a trailer, I would recommend hwy 191 from I-70 to get to Moab.
If you get a chance, don't miss the Colorado National Monument. You will have to find a place to leave your trailer though before visiting the monument. You can take exit 19 (in CO) from I-70 and turn South. There is a State CG and also a commercial CG as well as a shopping area just after you leave the hwy.
Enjoy your trip!