When I bought my new to me PuP last year I had this exact issue. When flushing all I heard was a click click click. I called Thetford (who has incredible customer service, the rep stayed on the phone with me for 50 minutes while I fixed it)
I was instructed to disconnect all power to the PuP and remove the toilet paper holder. This provides access to the water tank, then pull out the submersible pump motor. Then was told to lube "the heck" out of the motor with WD-40 and try to spin it manually.
Sure enough my pump motor was frozen, using a screwdriver and liberal amounts of WD-40 I sprayed and turned it until it spun freely. Then reinstall everything and Presto, it works like new!!
I hope this helps :-)
Update * I was so excited that I knew the fix, I missed your last post that you resolved the issue already..... Sorry
2014 Jeep Liberty & 1994 Jayco 1207