OT: What type wood between frame rails and truck box?

[BRENT in 10-uh-C]

This is kinda off-topic but I really don't have a good place to ask this type question, so hopefully someone here knows the answer.

We are building a larger 'tow truck' to pull our trailers (-which will be used for hauling Model-As and parts ) and with the purchase of a Class 8 truck, we are now building a 11½ foot box (-see CAD drawing pictured below) that will effectively make this truck into a RV Toterhome. l have seen where a slab of wood is used between the frame rails and the box's steel frame to eliminate squeaking & slippage. Does anyone know what type of wood is typically used for this application? Would Ash (-since I have an abundance of that species at my disposal) be suitable for use in this area??

Thanks in advance!!

.