Everyone recommends spending a couple days with it in your driveway as if you were camping. You'll figure out what you need pretty quickly. I keep a running list going on my cell phone's Memo app of anything I need to buy or fix, so the next trip is always better.
Sewer hose length can be an issue in some parks. Most are close enough for a 5-10' section to reach. I've come across a few where it's been 30 feet away or to be able to position my RV to get the slides out, I can't go all the way into a spot, which puts the sewer hooks ups further away. On all these items you're asking about, I like to have as much length as possible, so I'm prepared for all situations. I have a 50' water hose, 30' of sewer hose sections and I'm buying a 50' extension cable for power. The electric cable extension is not really necessary in 99% of campgrounds, but if you are tailgating somewhere or at someone's house or something, it's nice to have that extra length to be able to plug in. Also, if you have a black tank flush feature, you'll want a separate hose solely for flushing the black tank. The last thing you want to do is use your drinking hose for that task. I'd also recommend a water filter of some sort. You never know what you're going to get at campgrounds sometimes. I stayed at one place that almost made me sick while showering because there was so much chlorine in the water. After that I was reminded why we had a filter for the old trailer and I promptly bought one. The $15 ones work fine for the occasional camper.
I also keep 4x6 blocks of wood for use under the stabilizer jacks and tongue jack. I think it provides extra stability when most spots are on soft ground. They also work well if you have the unfortunate need to jack up our trailer to change a tire. I went to Home Depot and bought an 8' piece and had them cut it up into four 1' blocks and two 2' blocks. Speaking of blocks, leveling blocks are a good idea as well in case you come across an uneven spot. Many people use 2x6 wood for this or you can buy the plastic ones that interlock.
I'd get a mat for outside the door. Between us and the dogs going in and out of the RV, the mat has saved a ton of dirt from coming inside. I haven't used it yet, but if you run into a muddy situation having clear Carpet Mask ($15 on Amazon) can be a life saver. It sticks to the carpet and protects it from stains and you can peel it off whenever you want. I have a roll at the ready for whenever I may need it.
As I have in this post, you'll keep thinking of stuff, but just enjoy it and have fun. No matter what you'll be more prepared than ever just having a new trailer to play with.
