Long dump hose (or a second one you can chain together). My first trip involved totally setting up at a site only to find out the dump hose won't reach the sewer. I had to hitch it all up and back up another 5 feet. I guess the real lesson is gauge the length of your connections before dropping anchor.
I bought most of my RV necessities from Amazon. I pretty much bought the top-selling RV items that popped up that I thought I would need: leveling blocks, chocks, water hose, pressure regulator, sewer hose support, and biopaks.