Are you going to charge extra for that?

[judway]

A couple of rolls of black electrical  tape will come in handy at some time later to fix the leaking sewer hose until it can be replaced at a good price.

 

[Rene T]

A couple of rolls of black electrical  tape will come in handy at some time later to fix the leaking sewer hose until it can be replaced at a good price.

Or Duct tape.

 

[BinaryBob]

I would highly recommend a pressure relator as previously mentioned. The only think is don't buy one of those $10.00 one's. They are junk and all they pretty much do is restrict the flow. You want something like this:
https://www.amazon.com/Valterra-A01-1117VP-Lead-Free-Adjustable-Regulator/dp/B003YJLAIK/ref=sr_1_1?ie=UTF8&qid=1486580713&sr=8-1&keywords=valterra+water+pressure+regulator
This one is adjustable and well worth the money.
Something else you may want to think about getting some day is a surge protector. Getting the right one, will protect all your electronic stuff if you ever get a power surge. Also a good one will pick up a electrical power problem at a campground power supply. This one is pretty much the best in the market. You can get a portable one or one that is hard wired.  Progressive garrantees their product for life if it should fail due to a power surge.
https://www.amazon.com/Progressive-Industries-EMS-PT30X-Digital-Protector/dp/B01N0W4CZ8/ref=pd_lpo_263_tr_t_3?_encoding=UTF8&psc=1&refRID=72BK3Z843Q94T9MWQCJB

Rene (as usual) great advise...
The cheap regulators usually included with a "start up" kit go in my bin for emergency backup.
The Valterra with gauge is my preferred regulator. I've stayed in places with 100+ psi water. (Duluth Lakehead Boat Basin). At least they alert you to that fact and insist on a regulator you can "rent". No thanks. Valterra is the way to go.

 

[Gary RV_Wizard]

I can't imagine that any RV comes without its basic shore power cord, even if only 15 ft or so long. Additional extensions would be extra, and you will probably need one sooner or later.

Seems to me some of the replies have gone beyond what is essential for start-up and strayed into the "nice to have" or even "elegant". I'm fairly confident you can survive a few trips without an icemaker or a doormat, but they are indeed good to have (at least now & then). Don't try to buy upfront everything you imagine you might ever want. You will waste money and drive yourself nuts. Get a few basics and  buy as you discover a need. It's smart to make your first few trips to an area that has a Walmart handy!

Much of what you will want can be bought at Walmar,t or on Amazon or Ebay or other online RV discounter. At substantial savings.