"My question would be if diesel pushers can do this, why can't a Class 8 motorhome?"

Opposing slides on diesel pushers have been in the market place for how long? One year possibly two years. What are some of these motorhomes going to be like in say 5 years? Just because it is being done by RV manufacturers doesn't always mean that there can't be problems. For years there have been examples of various companies producing units that are overloaded from the factory.

Showhauler and NRC have both built a fair number of truck conversions. Many with slides. There is something that makes them not want to build this kind of floorplan.