Well, I went to Lowes last weekend, in search of inspiration, and I think I made a good find, or at least we shall see over Labor Day. Anyway, when I saw the traffic cones, the light bulb went off, and I knew I had a great funnel. The narrow end should fit nicely down the hole, and hold the trap open for me. I did cut an inch or so off to remove the narrowing lip on it, and make the pinch point wider, while still fitting in a 3" tube. I'll have a PCV ram rod with me just in case. The wide end has a 7.5" opening, with a lip that makes it a full 10" target. I should be able to hit that much easier than the little drain hole at the bottom of the dump pad. The cone did have some nubs that looked like crap catchers, so I cut those off and smoothed them out. I think it should work quite nicely for anyone that is trying to pour a bucket/ammo can style groover down the RV dump station hole.
Shop Corsan 18-in Plumbers Safety Cone at Lowes.com