Executing an RPN expression is most easily performed using a stack. Push the operands onto the stack until you reach an operator, at which point pop the last two operands and perform the calculation. The result is pushed onto the stack and you continue to evaluate the expression as before.
I don't remember exactly which calculator I first had but it was very much like this and my "big" Christmas present in mid 1970s this is the HP-35 Calculator from 1972. Look at the cost! It is still my favourite calculator of all time. Great keyboard, and used Reverse Polish notation logic for entry, which seemed so logical.