If you compare the actual useable square footage of a MH and a 5ver with the same length and the same sized and number of slideouts the 5ver will have more square footage.
Here's why, the MH has from 1 foot to up to 3 feet of dash area.
As others have pointed out it depends on the floorplan layout as to whether the RV has or feels like it has more room.
For example the Safari line of MHs were designed with large walk way areas, extra room in the bathroom. The owner of the company was a big guy and wanted his MHs to provide plenty of room with no bottleneck areas (areas where two people couldn't pass each other). So when compared with other MHs the Safari seemed roomier and yet the square footage was the same as like sized MHs from other manufacturers.