Max trailer rating is just over 13,000# with a max GCWR of 19,000#.
Note that GCWR is the weight is the total when truck (fully loaded) and trailer are weighed and added up. For example, when your RV is hitched to the truck and driven onto a set of scales. Truck weight = 8500# and RV weight (on it's wheels) is 10,000, that equals a GCWR of 18,500#.
Look for an RV with about a 10,000# GVWR in my opinion. Length isn't much of an issue until you want to fit it in a campsite. weight is an issue any time you tow.
Here are the charts for 2001 models of Dodge Ram
CLICKY.
Here are the 1999 charts for all makes.