First, see what's included. Many dealers include a "starter kit", which includes a sewer hose (called a stinky slinky), a white potable water hose, and some other basics. You definitely need these two hoses. Pick-up an adapter to allow you to plug into 15 or 20 amp 120v once you get home. I would also get a surge supressor for when you are plugged into a CG - too many power fluctuations in CG's not to have one of these! Much of the other stuff that you will need will depend on your camping style. Try "Camp Driveway" a few nights and make a list of the things that you need from the house/garage. You can then shop on-line, the dealer, WalMart, or whatever convenient, economical location that you want.
For safe towing, you'll need a GOOD set of extended mirrors (if your tow vehicle isn't so equipped), for wheel chocks I'm a Rotochock user, and these need to be ordered by calling Rotochock directly. Don't bother with the $5 plastic wheel chocks from the dealer - they serve little or no purpose.
Also, pick-up a small to medium size tackle box. Fill it with spare fuses, bulbs, solderless connectors, electrical tape, tie-wraps, garden hose gaskets, assorted nuts, bolts, washers, screws and nails, duct tape, Marvel Mystery Oil, WD-40, and of course, some duct tape. Every other month or so, Harbor Freight has a cheap electrical meter as their "free tool" for visiting the store. Wait until they have this (watch the Sunday coupons) and pick one of them up to stay in the TT.
I could keep going, but others may want to chime in.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter