Usuallt the model tells you the inside length of the unit, but this isn't a stead fast rule, so the best thing to do is measure your trailer now, then measure from the hitch ball to the center of your rear axle on the truck. add the two together and that will be the outside length of a fifth wheel that will give you the same overall length.
05 Alfa Gold 40' Motor Home "Goldie",
03 Malibu Toad
in a 24' CargoMate trailer.