I have used the 4" drop 5000 lb model with an 86, 25ft Sovereign
. Tongue weight is around 700 lb. Worked fine for us. It is extremely robust and the welds are first rate. You may wish to use the higher rating just to have a bit of a safety factor.
Only issue is it puts the pivot point a little further behind the rear axle. This did not cause any sway issues as we were using a 3/4 ton Dodge Ram diesel with extended cab with an 8ft bed. Truck had a very long wheelbase and was extremely stable. Our electric umbilical cord was long enough as well.