Unfortunately they seem to have abandoned it on their newer models: I recently got a HP Prime and no RPN in sight afaict (https://www.hp.com/us-en/campaigns/prime-graphing-calculator...) .

Sure there is. Go to Settings and move down to Entry. Hit Enter and you get your choice of Textbook, Algebraic, or RPN. Go down to RPN, press Enter and then Esc.

Presto, RPN.

Thank you - I used the calculator more in the last hour after making this change than I had in the past month. I'm ashamed I didn't figure this out - I should have done a better job of RTM.