20 inch vs 22 inch rims - any difference to towing?

Larger rims will usually have lower weight ratings. Most HD trucks come with 18" rims for a reason.

If you stick with the same overall tire diameter, meaning you'd have to go with a lower profile sidewall, you won't see any degradation in performace but the ride will be harsher. If you upsize tire diameter you will see a loss in performance as you are effectively changing the gear ratio.

With 22s you will also be much more limited in tire selection and they will cost more.