Creak when reversing the fifth wheel

I prefer Red and Tacky for grease. There is a grease zerk on the underside of the hitch head, and the rubber supports for the head should have some grease put on them. Also a little grease on the inside of the jaws where they contact the pin is a good idea. Is it a slider hitch? Have you checked the tightness of all the bolts?

However, I think your problem lies with the trailer. You don't say what it is, what size it is, or how old it is, but I would be checking the suspension if I were you.