The guide rod is the rod that is mounted parallel to the hydraulic piston that lifts the top. Look near the curb side wheels and you will see it, it looks like a long piece of smooth rebar. It must be kept clean and lubed, if not you can risk bending the piston. Periodically clean the rod and put any good grease on it. While you are under there, clean and lube the cables and pulleys. Check for any wear or fraying.
There are many posts about it here. Look at:
Hydraulic lift system - Hi-Lo camper travel trailer forum
I should add that there are manuals listed here that can give you some instruction, it doesn't have to be year or model specific.