Taking my "John" apart tomorrow for the same exact reason; "suddenly" low flow.
(Crappy job for a Sunday).
I have a Thetford Aqua Magic V and the flush valve has a sealed port on it to receive a hand flush wand.
I ordered the flush wand kit when I ordered the new valve.
Stay Tuned. Film at Eleven...
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW