There is some history involved here, so bear with me.
A couple decades ago, the front cap assemblies of many gas motorhomes (and a few diesel pushers as well) were not well designed and the coach chassis were not very rigid. When the front wheels came off the ground, the direction of force on the chassis changed )from push-up to pull-down) and the front ends moved in ways the coach builders never anticipated. Windshields popped out or cracked, molded fiberglass caps broke, and the cap seam at the roof opened up. The wheels didn't have to fully come off the ground for this to happen either. From the mid-90's thru early 2000's, this was a common complaint and another piece of conventional, popular wisdom was born, i.e. "Never lift the wheels off the ground". But at the same time the chassis were getting stronger and more rigid, e.g. the 2001 Workhorse W-series chassis with 50,000 psi steel) and the coach makers learned they had to make stronger front cap assemblies, so the problem went away in newer coaches. The coventional wisdom remained, however, if only because there were still a lot of coaches around with the structurally weak designs.
If you own a coach on a Workhorse Wxx chassis or a 2006 or later Ford F53 chassis, odds are you will have no chassis problem with wheels off the ground. Ditto for most any diesel pusher using a Spartan, Freightliner, Roadmaster, Peak, Dynomax or Gillig chassis.
The park brake thing also needs a bit more insight. It's true the PARK brake is rear wheels only, so lifting the rear wheels defeat that. But lifting the chassis on hydraulic jacks places a really strong anchor onto the ground, arguably a better parking brake than the rear wheels. As long as the jacks don't physically fail, anyway. But some coaches had knee-action jacks that sometimes suddenly folded under severe side-pressures, and some coaches had undersized jacks that failed for the same reason. So again, conventional wisdom was reinforced.
And of course, many RV owners are almost clueless about the mechanical equipment type or capability on their coach, so they they are best-advised to follow some simple rules to avoid any possible mistakes. "Don't do it" is about as simple as a rule can be!