I absolutely love RV slide-out storage. It makes RVing much easier and helps you stay organized. Commercial grade slide-out trays can be expensive but you can build them yourself for a fraction of the cost.
My friend and fellow RV enthusiast, Bob Rozek, wanted to move his freezer into another compartment under his motorhome to make room for his new toolbox. He already had a sliding tray where he put his new toolbox, but the compartment where he would move the freezer didn’t have one. 
He decided to use the Accuride accessory platform kit model 9301. These platform bracket kits are available in 12, 16, 22 and 28 inch lengths. Bob ordered the 28 inch model. You can view this product here: Accuride. Vendors who carry this system are easily found by using your internet search engine.
These sliding rails have locking stops which are critical for RV usage since they prevent the tray from sliding while driving.
Bob used 3/4 inch plywood for the tray and base supports. 
He used a powered rasp to make indentations to conform to the ridges on the rail slide.
This procedure allows for a tight fit for the tray.
Next he attached the base plates to the tray. The plates are made from 3/4 inch plywood strips about four inches wide. The base plates raise the sliders so they clear the lip of the storage compartment door frame.
He checked to be sure everything fit properly and drilled holes to attach the base plates to the floor of the storage compartment.
He checked to make sure the freezer would fit nicely, then removed the tray assembly and painted it black. He also put carpet on the tray to help steady the freezer while in motion.
You can install a tray system like this in any type of RV that has outside storage compartments. This system will hold between 350 to 500 pounds depending on the configuration.
Sliding more RV ideas your way – Jim Twamley, Professor of RVing
Smokey Camping 