The van/truck cab and chassis vehicles are crash tested like pickups and vans are crash tested. Also, the complete vans that van cab/chassis vehicles are derived from are crash tested as well. They all have to meet NHTSA and USDOT safety standards before they are approved for sale for use on US highways.
No, the completed class C motorhome coach is not crash tested. However, the vehicle upfitter company that finishes the incomplete cab/chassis vehicle into a completed motorhome must certify that it meets all applicable standards and there are standards and procedures that must be met when modifying anything on the cab/chassis part of the vehicle during the completion process.