Byrdman,
I own a 2006, 30 ft Holiday Rambler 5th wheel and sometime tow an 18 ft Searay boat behind it. Ken is right, it is not legal to double tow or triple tow as some states call it, in all states. A good website that has information for each state is towingworld.com and then click on towing laws. States that do allow it also have a maximum combined length of truck, 5th wheel and trailer.
I had a local welding shop that specializes in installing a hitch on a 5th wheel and paid extra to have it made heavier as the boat weighs 3500 lbs. Total cost for the hitch was about $800. I am using a standard 2 inch ball into class three receiver. Another limitation is how heavy the frame is on the 5th wheel, the welding shop told me the maximum I could tow would be 4500 lbs but 3500 is enough for me. Also, the boat trailer has surge brakes on it so that helps to minimize the stopping distance. I have been towing the boat occasionally for about 6 years now, sometimes has far as 2000 miles in a trip, and have had no problems but I do limit my speed to 60 mph and try to stay on interstates since the length of the total rig is 70 ft. I prefer to stop at truck stops for gas because of the longer turning radius. Also keep in mind that it is almost impossible to back up except for a short distance.
I hope this information helps you in your decision.
5thWheeler