Hijacker Jack Extensions

If I were doing it (and I tend to do these sort of things) I'd first reinforce the little circular jack leg foot pad by welding on some small gussets on the top side made from little triangle shaped pieces of steel. They have to be only an inch or two high so that the jacks still retract all or most of the way.
Then I'd use heavy steel pipe maybe a foot long as the extension. I'd weld a thick plate to the top, then drill and bolt it to the original jack feet. If the original feet aren't big enough I'd weld bigger ones on first. Or if you don't mind them being semi-permanent you can just weld 'em on... but I'd bolt them on in case you ever need to load the camper onto a lower truck. Then weld some plate to the bottoms of your extensions.
If you haven't got the tools to do that and haven't got a buddy who does, a welding shop can do it.

While theres a bazzilion jack feet that could be adapted, matching tube OD most require cutting off existing foot-
https://www.etrailer.com/dept-pg-Accessories_and_Parts-sf-Camper_Jack-pt-Jack_Foot-co-2_Inch_Diameter_Tubing.aspx

Happijac sells bolt ons that might work (not cheap), and easily duplicated, simular to what burningman described.
https://store.lci1.com/530000-6-inch-jack-extensions-set-of-4

Actually those look about right I didn't know they made those!
At that price unless you have all the material and tools lying around already I'd just buy those.