For a fifth wheel, the rule of thumb is to have 20% to 25% on the hitch.
Travel trailers are much less hitch weight, and should be between 10% and 15%. So if your maximum you "Should" put on the hitch is only about 500 pounds, it would limit you to a 5,000 pound trailer.
Remember that say you hitch up a 5,000 pound trailer, and then apply the 510 pounds of hitch weight, and tighten the hitch WD bars, some weight will transfer from the tow vehicle to the trailer axles. Your trailer weight might increase by as much as 80 - 100 pounds once hitched up.
in this exaple, the trailer weight went from 7840 to 8,000 pounds once the WD bars are tight.
http://forums.woodalls.com/Index.cfm/fuseaction/thread/tid/26921756.cfmFred.