I had the same issues pumping out initially until I found Alex from Nikimat's page. I used that design to make what my wife and I affectionately call the "poop straw". We used this fitting from Defender:
Deck Fill Diameter: 1-5/8 Inch
Actual Thread Diameter: 1-1/4 Inch MPT
Threads / Inch: 11.5
Model #: 310343503
I simply screwed the fitting into 1.5" ID Sanitation Hose without any sealant or adhesive. We store it in a garbage bag in the port side deep locker.
This enabled us to pump out. However, we still struggled to get a good seal between the pump nozzle and the fitting on the "poop straw" mostly because the fitting did not sit flush with the deck due to the length of the hose. I decided to inspect our tank design and found that the bottom of our holding tanks are only about 17 inches below the deck. The drain hose from the bottom of the holding tank to the ball valve is obviously much further below the deck but getting the "poop straw" down in that hole was not easy and made the fitting not sit perpendicular to the deck. I decided to cut the length of the "poop straw" to approximately 17 inches below the foam seal on the fitting. I also made sure to cut the bottom on a bit of an angle. The fitting now sits flush with the deck and I can simply press the pump into the fitting to get a good seal and can pump out with ease. It doesn't empty the tank 100% but it does get the vast majority out and it now takes us no time and the process is significantly cleaner too. I will add a new album called "Holding Tank Pump Out" with some pictures of our "poop straw" to the photos section.
Hope this helps.
SM #440 Cara
Deale, MD USA