Stupid question but we have recently hired in some Man xxl units, however they have a sliding 5th wheel, unfortunatley I have never used them before as our units have fixed wheels, transport manager don’t know either. So how do you use them?
It will have an extra 5th wheel style handle which will operate a scissor type of locking mechanism. When you drop the trailer you will be able to see it.
To operate it, ensure it is well greased,
hook up to an empty trailer but don’t hook up the suzies,
pull the lever out and lock it in place (exactly the same as a 5th wheel),
move your wagon back or forwards till the 5th wheel is where you want it and then release the lever,
shunt the wagon backwards and forwards gently, till the locking mechanism locks into place,
You may have to push the lever home, lock it in place with the dog-clip, finish hooking up your trailer, and do whatever you want to do with it.
Usually it isn’t nessesary to move the 5th wheel. It should be a bit in front of your drive axle on a 2axle and a bit further forward on a 3 axle unit. Move it far from there and you could be in trouble with overloading an axle. The further forward it is, the more weight is on your front axle and obviously, the further back it is the more weight is on your rear axles. There is a fairly central point where the weight is spread correctly over your axles for traction/ steering and legal weight distribution 
.
The other thing to be careful of is, if you move the 5th wheel forward with a shallow king-pin, then do a trailer change to one with a deep king-pin. You can easily end up with a - front of trailer/rear of cab interface situation. 
Also, if its a change over and the shallow king-pin trailer is taken away before you realise, it is a real pain in the proverbial to move it manually.
all the above is rot 
the reason for a slider is so the trailer can be put back as far away from rear of truck so you dont get grease on your designer skids and reeboks
hitch:
all the above is rot
