I would buy two 10-ft hoses. Get good supple ones. The cheap hoses get brittle too fast and crack. Also get caps for the hose ends to reduce messes and smells.
As @Binghi said, two 10 ft hoses are a good choice. You'll also need an elbow for the connection at the campsite. Either of these kits contain the key pieces. There are several other similar choices.
Might want to consider getting a dedicated water hose for the black tank flush connection. It probably wouldn't seem too obvious until you use a dump station.
20 feet of hose seems like about 10 feet too much. In over 10 years, I have never needed more than one 10 foot section of hose. There were a few times when I needed to use nearly every inch, and there were a few times when I had to jockey around a bit to get close enough, but I have never encountered a situation where 10 feet did not suffice.
I have 15 feet which has been plenty. If you are at camping sites with a sewer sump provided you should usually be able to position the camper within that distance
Susan & Bill, Yarmouth, Maine 2024 T@B 400 Boondock Black Canyon 2024 Kia EV9
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You will need a clear, 90 degree bend. Without the clear section, you won't know when it's done.
"Just Enough"
2024 T@B 400 Boondock Black Canyon
2024 Kia EV9