Look for hidden areas in the RV. For example in ours I found a large space (1foot X 2 feet) off to the side of one of the storage compartments. It was used to run the heating duct, some electrical and plumbing but there was still room for stuff I don't use often. Another spot I found was under the step leading up to the bed. I pried the tread off and there is a compartment perfect for my flashlights, flares and other breakdown supplies. I put a hinge on the tread so I could raise it up to access the compartment.
Under the RV is lots of available space. Have some custom steel boxes built or use some of the pre-made boxes used on utility and semi trucks.