I have a '97 Reese 16K manual slider. When new it wouldn't slide without taking weight of the hitch. I had it gunked up with white spray on grease.
I disassembled it completely and cleaned it up. Then sprayed the square tubes and rollers with a dry spray lube several times. Worked like a hot knife through butter. I spray wash the hitch in the spring and use lots of dry spray 2-3 times a summer. The hitch still slides easily without using the landing gears.
I stopped using the white spray grease as it attracted grit/dirt/wood chips/etc.