I have read somewhere that the number like 3505 stands for the first 2 number meaning it takes the same room on a campground lot as a 35 ft TT trailer.
Since a 35ft TT trailer is measured from the ball to the rear bumper then the 35 on a 5th wheel is the measurement from the ball of the truck to the rear of the trailer. So my 3500RL is actualy around 39ft to the pin. but if I measure from the ball of my truck to the rear it will be close to 35ft
Also meaning that the front of my truck to the rear of the trailer is the same lenght as if I was hooked on a 35ft TT trailer.
I checked this with my neighbour's 34ft trailer and he concluded he needs a 5th wheel because his has 30 ft of living space and mine has 38' 8" living space, for the same manuvering lenght.
When I register at campgrounds I alway call the lenght of my trailer as a 35ft trailer. they don't come out and measure it anyway.