I typically do that for additional power outside - and as Rusty said - as long as pedestal is wired correctly - it works fine.
(Worst case should be tripping a campground breaker - in your pedestal or upstream... never had it happen.)
Haven't run into the 'single plug rule' yet myself.
I use it for occasional power tool use and induction cooktop - instead of running thru the RV.
Of course any line surge suppression/monitoring you have on the RV doesn't apply to whatever you plug in directly to pedestal - so you are exposed to power surges, voltage drops, etc...
Make sure the extension cord gauge and length is up to the task for what you will be drawing thru it - as the protection is at 20amps... Many 'orange' extension cords are 15amp at best.
The circuit breaker is there to protect the campground wiring - not what you plug in...
The 20amp pedestal outlet should be GFCI protected - so that will protect you in wet location from ground fault.
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