What year and model of Excursion do you have, and what model of truck are you plannng to haul? I have a friend who tows a RZR on top of Chevy 3/4T 4X4 LB diesel and it works fine. However, I wouldn't put the RZR on top of the truck and then put both on a car hauler. He bought the Chevy because it could be towed four down without any driveline modifications.
On my 2005 39S, the receiver is not marked with a weight capacity so I called Fleetwood. I was told that the 2005 receiver was only rated for 5,000#, but it could be modified to 10,000# by having the existing reciever cut out and and a heavier duty one welded in. The 2006 Excursion 39S is rated for 10,000#. The frame rails are the same, they just welded in a heavier receiver. I'm only towing a Jeep Wrangler which I knew is below 5,000#, but I wanted to know what I was rated at.
Bill