The W-C Newport was a disaster from the day it went on the market. Thetford discontinued it about 10 years ago, along with the rest of the entire W-C product line, so no tech support is still available. I have what passes for the owners manual--which I'll be glad to send you if you'll send me an PM that includes your email address (no way to attach anything to a PM). No trouble-shooting guide, but at least you'll have a parts list and exploded drawing. Unlikely, though, that any parts are still available. You can try calling Thetford at the phone # provided in the owners manual, but don't expect much help from them for any W-C toilet.
So IMO it's time for a new toilet. The good news is, almost all of 'em are available as a "conversion" (everything south of the bowl), which will allow you to re-use your bowl seat and lid. My choice would be the Raritan SeaEra Raritan SeaEra Conversion Promo Sheet, available in both sea water and pressurized fresh water versions. Defender has it: Raritan SeaEra "conversion" at Defender
Peggie "If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein