How to know the actual lenght of Fifth Wheels

You will never know what the actual overall length is from the brochure/online description. The true way is to do an actual measurement. If you find a floorplan that you like, get on that manufacturer's owner's website and ask. People are more that willing to go out and measure their rigs. Also, once you have model numbers narrowed down, post back here and there are bound to be some owners on here. Good luck in your search.

To be accurate the only way is to measure and then YOU will know

Several fivers have front caps (nose cones) that extend forward of the hitch. If storage is completely enclosed, this must be factored in to get a true length.
I have dealt with this issue lately. Drop a plumb line from forward most part of trailer and mark the spot on the ground, do the same for rear most part, borrow a 50 ft. tape and measure. This will give you the true length, and this figure will not match up with many manufacturer's specs.

Old Guy, That is what I am referring to. What we have is a lot in an ESP park that only allows a 40' rig. I need to buy only a 40' rig not an inch longer. But of the desired mentioned type RV there is no way without measuring each and every rig.

are you referring the length as in storage wise? do you need it to go into a certain space? if so then the measurement will be from the forwardmost tip of the hitch to the back side of the bumper or spare tire.

Only way to know for sure is measure it. I know the Montana web site says the stated length is from the center of the king pin to the rear bumper. But that leaves a few inches in front of the king pin. And probably another few inches for the ladder extending past the bumper.