Found a real service station (an actual shop) that sold PP and they knew exactly what to do. He unscrewewd the brass coupler from the fill hose and used it to relieve the pressure in the tank. It took several minutes for the pressure to disapate. After that he filled it in the regular manor by loosening the bleed screw.
No wonder PP wouldn't go in, there was a LOT of pressure in there.
He went on to explain some have pressure and some are "pre-purged". The latter are under a vacuum and ready for filling.
Thanks everyone for all your input!