My under sink area has plywood underneath the paneling. The thin paneling under the upper cabinets is double, there is a hollow space between the same paneling inside the cabinet. This allow for wires. I'm sure you can have solid wood behind all the structural walls, but the upper cabinets are not made for heavy items, the lower cabinets are.
Scamps use OSB board in their campers which crumbles when it gets wet, Escape does not use any OSB, all wood. Wood paneling has been made with paper facades for years as solid paneling went out in the 60's due to cost. Scamp Deluxe models has a nice birch solid wood interior, similar to the old time "tin cans" with nice wood interiors.
never in doubt, often wrong