The backflow is NOT coming from the holding tank. The inlet fitting on a holding tank is at or on the top of the tank...you've said there's very little in the tank, which means tank contents would have to jump up to the inlet fitting to run back to toilet unless the tank is lying on it's side. So what IS running back is waste that never made it into the tank, is left sitting in the toilet discharge hose.
The wet/dry mode has nothing to do with OUTbound waste in the bowl...the WET mode, allows the pump to pull flush water IN, the DRY mode blocks incoming flush water.
A brand new joker valve (yes, that IS what it's called...I don't know why either) will prevent back flow, but only till enough flushes have gone through it to stretch the slit in it. At first it's slow seepage, but the more flushes that go through it, the wider the slit gets...until it finally becomes a hole. If it's never been replaced, it might as well not even be there.
If your toilet is original on a 10 year old boat, it's 10 years old...the average life of a Jabsco manual toilet pump is 2-5 years, so I suspect the pump has been replaced at least once. A replacement pump is inexpensive (forget installing a "service kit"...it's a PITA to do and the price is 85-90% of the price a new pump) and replacing it is easy--remove the 4 bolts that hold it on the base, remove the inlet and discharge hose...bolt the new pump the base, replace the hoses...and it's done. There will be a new joker valve in the new pump...it should be replaced annually--or at least every two years.
At the risk of being accused of a little shameless self-promotion, I'm gonna second Apex recommendation to buy my book (see link in my signature). The title (my publisher's idea) is a bit misleading...'cuz although it does deal with every source of odor on a boat and how to cure, or better yet PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the laws, describes all the types of systems and how they work, and will help you learn how to operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is! And I'm always glad to answer any questions it doesn't.
--Peggie