We have a 25' 30amp extension but I only think we've used the extension once or twice since buying it. Still nice to have for piece of mind. 2nd white water hose and sewer hoses are used much more often so I'd get both of those as well. When you get a 2nd sewer hose look for one that comes with a threaded 90 adapter to connect to the drain. http://www.campingworld.com/shopping...ewer-kit/44151
Lots of options for storing those sewer hoses that you might want to consider. Some home grown setups you can find on this site and some store bought. We use the E-Z Hose Carrier mounted to the frame right next to our dump. Works great for us. 64" E-Z Hose Carrier- Black - Valterra A04-0153XBK - Sewer Accessories - Camping World
If you don't have a 30A Female to 15A Male plug adapter I'd get one for those sites that don't have 30A. Also, in a pinch with the adapter you can use a normal outdoor extension cord to get some extra distance if needed. Just won't be able to use you A/C. Power Grip Adapter - 15A Male to 30A Female - Camco 55165 - Electrical Adapters - Camping World
I'd outfit your trailer with a small tool kit specific to the trailer. Include a socket specific to the water heater drain and possibly one that might be needed to manually crank your slides in the unlikely chance that you loose a motor or battery goes dead. Throw in some replacement bulbs, teflon tape, fuses, multi-bit screwdriver.
Since this is a new trailer or at least new to you trailer once set-up with the water connected I'd do a walk-around to visually check as much of the plumbing as you can looking for any puddles that reveal a leak. We had one at the back of the outside shower connection. Connection was just a little loose and a simple turn fixed things up before any serious damage was done.