Are you gonna pull those pistols or whistle Dixie?

some thinner lumber or leveling blocks so you can adjust closer than 2 inch at a time. tent pegs and rope, awning mat, I like a propane diversion/quick connect for bbq or camp stove outside. plus all the homey stuff like toiletries, flash lights, cooking stuff, shower curtain, bedding, long list. how in depth do you want to go

Wait just a minute on that water filter! My TT, and I assume many others, came with a built-in, factory-installed water filter. You should check on that before you purchase one.

-Speak

I think many new trailers come with a starter kit, in a box. Mine did, with items such as fresh water hose, sewer hose, toilet paper, adapter plug, a few others I don't remember. My sales guy showed it to me, said mostly cheap junk, yes an honest sales guy, that went on to say I could keep my stuff from my trade, as it was likely much better than kit junk. The kit junk will work, until you get something better.

Trailer should come with a spare, or make them throw one in.

Jerry

Make sure they include a mounted spare tire... I was amazed when a friend bought a TT and had to pay extra for the spare. I always assumed a TT comes with a spare! Spare tire cover is nice, and you can paint it if it has an obnoxious decal / advertisement on it.

The TT should come with a ~25ft power cord and sewer hose. As others posted, you will probably end up buying extra lengths of power cord - I just bought mine from Walmart, so I have 25' and 30' cords - works well when you are just a few feet shy of a power outlet, or if the power outlet is on the "wrong" side of your campsite. The sewer hose will work for awhile, just buy a replacement and keep it in the camper until you need it. Ask if the dealer will include sewer hose adapters, including a 45 degree elbow (clear if possible).

Awning "deflappers" - clamps with velcro that attach to the awning and braces to help prevent the awning from being whipped around by the wind. Get a pair!

Depending on the dealer and how large of a store they have, ask if they'll give you some sort of discount card or credit for their store. As you get camping trips under your belt, you will figure out what extra stuff you want.

Try camping a few times close to home, so that you can return to get stuff that you find you need. Try not to buy too much stuff right away - most everyone does, though, and you end up with various things that you find you really don't use.

Everyone does things a little different, using different products. Take all this advice, and figure out what works for you.

You will also need (4) wheel chocks abd a clear 45 degree sewer adapter ( so you can tell when your tanks are empty)
And you'll want power cord adapters, so you can connect to 15A, 20A, 30 and 50A.

when you go shopping for that surge protector, make it a multi-function one, not a $90 special cheapie. and I would buy a couple of different length hoses, not just one really long one. and for a longer electrical cord I would buy a standard extension cord type, with the standard M/F 30 or 50 amp connectors on the ends. and not the $9 special pressure regulator, get a watts whole house one.
bumpy

Power cord should come with travel trailer. Most of the time, a cheapie sewer hose is included, but you will want to get an additional one for those times when you need it. Plan on buying a water hose and filter if you want to use a filter.
If you are going to buy a surge guard, you'll have to determine what you want. I use a Progressive Industries EMS system (30 amp portable for my trailer). For years, I didn't use one, but finally bought one when we bought the Minnie and I'm glad we have it as it has been useful several times.

A power tongue jack is also highly recommended. Some dealers include them and others don't. I wouldn't have a trailer without one, but it is a personal preference. Same goes for slide toppers.

You want to check carefully what packages of upgrades are available from the manufacturer, and review carefully the specs of any trailer at a dealer for what package(s) came with the trailer. A really stripped down trailer may not have a spare tire! And, some packages may be actually required by the manufacturer....which is kind of crazy because then it shouldn't even be listed as a package but just as standard equipment.

Try to get MaxAir vents installed.

You might get a black/gray hose, but whatever they give you will be the cheapest thing they have and not what you want.

You should get propane tanks, at least one battery, and possibly a spare tire (but no spare tire cover unless the dealer has one with their name plastered all over it so everyone can see where you bought (I pass on those :))

You will need to purchase
water hoses (potable) - probably at least 50 feet
power cords - look for the marinco marine style. 50 foot one and a 20 foot one. Size depends on whether you have 30 AMP or 50 AMP service. (for example, a 50 foot 50 AMP cord marine style (Marinco 50A Cordset 125/250V 50 Feet 6152SPP) is on sale now for $701 CDN at binnacle.com

check out a marina or goto binnacle.com for a lot of this stuff in addition to the rv stores - boats and rv's have a lot in common