One of the top quality rv manufacturers is Continental coach. They use a combination of tubing and I beams. Their total frame depth ranges from 18 to 22 inches deep. As posted earlier DRV uses stacked tubing for a total of 15" depth. Augusta RV uses a 12" I beam with a 2x2 tubing tuning underneath. My Citation has an 18" deep C channel frame with 2x3 tubing welded underneath where the axles attach. My brother in law has a Phoenix 5th wheel which uses a 10" I beam frame as do some Cedar Creek 5th wheels. Are you purchasing a new or used 5th wheel?