Sliding a fifth wheel forward will add weight to your steers and takes weight off of your drives.
Sliding it backwards will take weight off steers and add weight to your drives.
A tip: Always lower your landing gear when sliding a fifth wheel. It makes it much easier on you and the truck.
When done, make sure your fifth wheel pins are locked in place and of course crank up the landing gear.
As for the trailer, sliding tandems to the rear will add weight to your drives and sliding them forward will add more weight to the trailer axles.
Tip: Make sure all pins have locked in place on both sides before moving the vehicle.
I hope this is of some help.
Please everyone if you see a driver struggling with this or backing up lend a hand. We all have to work with each other out here; lets make it as easy as possible.