OEM jacks are not supposed to carry much weight. They are for fine-tuning the level, and also for stability, to reduce bouncing up and down on springy wheels.
Snowbirds typically load jacks more, so it starts carrying more weight, since they are living there and don't want any bouncing at all. Though many put additional jacks near axles and/or cement blocks, to take load off their RV jacks.
I never store my MH on the jacks. I don't think storing on just the tires does any harm - hasn't in four years so far.
I'd prefer not to have all that weight just sitting there on the hydraulic system for long periods of time. Yes, I realize that when a snow bird goes some place for the winter, they'll often sit there on their jacks for 4 - 6 months at a time without any harm - but that's my preference. Sort of a Ford and Chevy thing.......