These things change all the time. The dealer supplies the battery and should have the information you seek, ask someone in the service department as they will know a lot more then the sales people. All you might need to buy is a sewer and water hose but some dealers supply these too. If you buy a water hose be sure to get the white ones that are for potable water. The furnace, refrigerator and water heater all run off 12 volt and propane. The wate pump is 12 volt and there is usually a switch near the monitor panel. The water heater may have an optional 120 volt element and the refrigerater also runs off of 120 volt in additon to the propane. Most of your lighting will be 12 volt and run off the battery but there may be a few 120 volt lights too, usually in the dining area. The slideouts operate off of 12 volt and usually require a well charged battery to operate.
The air conditioner and microwave will be 120 volt and there will be 120 volt outlets throughout the camper. Depending on your tow vehicle you may need a weight distributing hitch. This hitch will help take some of the weight off the rear of your tow vehicle and redistribute it and is almost always advisable to have. Here is an example,
http://www.etrailer.com/Weight-Distr...s/PS49903.html Some type of sway control is usually desired too and a friction sway control can be seen on the set up in the link.
The toilet is plumbed to the black water tank and sometimes the bathroom sink, but the sinks usually go to the gray water tank. Make sure the black water (sewage) tank is at least 3/4 full before draining, don't leave the valve open even when hooked up to sewer, you can leave the gray water valve open if you have full hook ups but most people like to have some gray water to flush out the sewer hose after dumping the black tank. Good luck with your new trailer.