2/3s should be plenty. Now I'm curious. Only problem I've ever seen outside of plugged vent or product blocking the pipe, is what OC mentioned (the cut out) or the drain pipe actually extending too far into the tank. On a Wildcat, I had seen where the pipe went into the tank and was only a few inches off the bottom. So, product piled up quick. That was a factory fix for them. Tank bottom to pipe is easy to check with an old awning rod with the 90 deg bend on the end, IF you have a straight shot, but yours, and mine both don't have that straight shot. I'd be looking into the camera / scope option to investigate a little further.
How quick does it drain when you dump?
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.