I hav emailed the 521 folks and they do not have a model for 120 volt operation- I haven't measured the back emf voltage on a running unit though. The Supco SPP6E is a relay based unit, and while Supco used to spec it for 90-270 vac, now they spec it at higher- and don't recommend it for 120 volt units. I'm not sure what changed.
Dometic does make a potential relay based start kit- I'll look up the number. It is specifically meant to run a Penguin low profile unit on an Onan 2800 microlight.
Here's the Dometic instruction sheet for the relay based kit-
http://bryantrv.com/docs2/docs/quickstart.pdf