The initial "whoosh" of water shouldn't take longer than 30 seconds (if that long) for either the black or grey. If you want every last drop out (which isn't necessary), yeah, you're gonna be standing there a while. Once the water slows to almost a trickle, you're done. Close the valve and get on the road or back to camping.
I think the grey tank outlet dia. is 3". It would take a sizable blockage to stop it. However, As you discovered, there very well could be a layer of material build-up still inside from construction, causing it drain slower when the level gets closer to empty.
BTW, I had the same problem with the blade not fully closing on the black tank. Some debris (ABS shavings and whatnot) partially filled the groove where the blade seats, causing it to not seal.
2014 Northwood Wolf Creek 850
2005 Ford F350 SRW SuperCab/LongBed 6.0 Powerstroke
QuickTrick's Towing Tune
Torklift Tie Downs/Fastguns/Upper/Lower Stableloads
Rancho 9000's