I use batteries, a 120vac generator, and a 12vdc generator, all depending on the conditions I'm going to be dealing with. There are advantages and disadvantages to every option as has been posted.
Ray_WA's post is absolutely 100% correct. You need to determine how many watts you're going to need to be able to supply at any given time. Then you will need to figure out how long you will need to supply that power. That will arm you with an amp-hour figure necessary to calculate your power needs. I think it will be surprising to you. A difficult lesson I learned a long time ago and I think many others on this site have learned is about energy conservation.
It's trendy to be green currently, but it's a whole different ball game when you NEED to be because of practical limitations. It certainly will get you thinking about alternatives. Point and case might be something as simple as coffee. Electric drip coffeemakers are a staple in most households and can be had for under $10 but on the road they aren't such a great, cheap option. Those suckers draw some serious wattage! You might soon find that a tea kettle on the propane fired stove using either freezedried coffee or a french press will be quicker and less taxing from an energy standpoint. You might even get a taste for iced or lukewarm coffee.
I don't mean to scare you. It's just a reality we all must face. I don't think what you're proposing is anything extreme. Heck, it pretty much describes my energy uses. If I was just running my lights (all fluorescent for reduced power usage
), my TV, my PS2, and my little surround sound system I think I could go for 2-3 days on battery power alone. But I also like to keep my food from spoiling which means a power hungry fridge.
Calculating the load you want to support can be a daunting task. If you were to give us more information about length of stays, exactly what you want to operate, etc (essentially your vision for the bus) then I think we can better help you estimate what you need.