Update:
Pulled the aft head this weekend and installed all new seals, impeller, and joker valve. Also THOROUGHLY cleaned everything while it was apart. Wow, what a difference! No more backflow and it actually smells almost okay in the head
Next step is to do the same for the forward head and then begin the hose replacement process.
For anyone looking to do this job on their head, this is the kit I used for my Jabsco 37010 toilet:
Jabsco 37040-0000 Marine Marine Electric Toilet Service Kit (37010 Series)
Amazon.com : Jabsco 37040-0000 Marine Marine Electric Toilet Service Kit (37010 Series) : Boating Heads : Sports & Outdoors