Sewer Slinkies are "iffy-iffy", I guess it just depends? It depends upon how far from the ground your sewer outlet is on the RV and it depends upon how far off the ground the top of the campground sewer pipe sticks up. On my last camper, at one particular campground, because of the fall of the ground and the height of the pipe in the ground, I had problems with enough down hill slope drain. When I dumped the tanks, I had to walk the hose to get everything to drain. One of those slinkies under it would have been a nightmare.
Still, I've seen many campers use them successfully on more level terrain or even when the ground pipe was down hill slightly.
I've been the owner of RV's for quite a while now. We've had 1 pop-up and 3 travel trailers, and still have not purchased one of those slinkies. Probably never will either.
If you want to get anything for the sewer hose system, get a 4 or 6 inch section of clear extension. You connect to your camper and the hose connects to the other end. When flushing the tanks, you can see what's coming through. I think you'll find this MUCH more beneficial! I use mine 100% of the time at full hook-up campsites and at dump stations. (They don't cost very much either).
Here's some "little" things you might think about to help outfit your new camper. You may think, "what will I do with that", but once you're at a campsite, you'll find dozens of uses:
A bunch of various sizes of bungee cords.
A couple 25 foot utility style electric extension cords.
A 50 or 100 foot of light weight rope, (not like a ski rope for a boat - too heavy, not needed), something light, 100 pound test or less. You can use it to cut into different lengths and won't feel bad about cutting it. You'll be surprised how many items you'll be tying down on windy days.
A decent camping hatchet, that can double as a hammer ... priceless!
A small carpenter's level (like the 9 inch ones)... why? Well, it comes in handy for a lot of things. Not just to help level the camper, but if you decide to get a flat griddle for frying eggs on the picnic table, it's real important the griddle is level all directions. Otherwise the eggs will drip away before they get solidified by the heat! (and pancakes too!). You'll find dozens of uses for a level.
I know these items are not "camping glamorous" items, but they are very functional items that you will want and need eventually.
The list is gigantic of all the things you "could" get, everything from dedicated camping clothing, to new pots and pans and dinnerware dedicated for the camper. Tools, games, you name it.
But of everything I have in the camper, the items listed above are the ones I use all the time, every time!