Rick, I’m sure you thoroughly rinse your pieces after each step
(dishsoap, rinse, ultrasonic, rinse)…I would think that the
tarnish is then the result of “laying them out to dry overnight”.
The tarnish on the back is a clue! It sounds like your problem stems
from the jewelry laying on a damp towel. I get the best result by
holding each piece under a blow dryer that is set on low for a minute
or two to get rid of the majority of the moisture; then, I lay them
out on a DRY towel overnight to give the rest of the moisture time to
evaporate from the crevices. You may also want to try anti tarnish
strips in the plastic bags. Hope this is helpful!