This is not some internet idea or ideal speculation. The manufacturer has provided specs. Greatly exceeding those specs is a poor idea and can result in an unsafe rig with poor handling and overloaded tires, wheels and suspension.
Half ton pickups are great for hauling some sacks of fertilizer or plywood from the home center but most are not really set up for any heavy duty use. When the RV salesperson states a warning, I would take it really seriously and listen to the options they have as well as checking all the specs.
