I think as long as you also attach those 18" wheels to a 3/4 or better yet a 1 ton truck and not an F150, that would be better...
I'm towing a 26', #7000 GVWR TT and it's all I want to tow with my F150 and it's even setup with MaxTow and has over #1900 payload!
It's not so much the size of the wheels. It's the size of the tires you have. Those 22's you have have small sidewall tires where the 18" wheels will have taller sidewalls so the overall tire height of both sets are going to be about the same.. I've got OEM 20" wheel/tire setup, but my OEM spare tire is on an 18" wheel, but the overall tire height is roughly the same.
Anyway, good luck with whatever you do! Mitch