I went to Block Island last year for the first time on my 25' sloop and also just had a small sani poti - no real holding tank. I am sure the pump out boat could do it for you but what I did was just detach the holding tank part of the sani poti and brought it in on the dink and emptied it out in the porta johns they have all around the marinas. I only had to do it once and it was very clean and easy to do. We also just anchored out in the Great Salt Pont because we were there last year in the height of the season and it was pretty busy. Unless you have a lot of rode, anchor to the sides of the harbor because the Great Salt Pond is very deep in the middle and usually the larger boats anchor up in the middle.
Now that I have a new, bigger boat I can't wait to get back to Block this year. I'm planning on going end of August. Have fun on your trip!