A couple of years ago I grew 4 acres of pumpkins ... what a nightmare, there were thousands of them!! Somebody that was buying a few told me about washing the outside of the pumpkin with a bleach solution to keep them for a long time, but I never looked into it further.
The first year I grew pumpkins, I planted them way too early. They matured in late summer and I drenched them them in a solution of 10% Clorox and 90% water. I stored them in my shop (heated to 50F) until Christmas.
I have since waited to plant them until the first of July, and they mature before frost, and store well until Christmas without treatment.