Personally, I wouldn't consider just 110 only. I'd go with 12V only if I had to choose just one. At least, I'd have lights and the fan no matter where I went or where I stopped. That said, I also wanted an A/C available whenever I had 110V input, and that will be nearly everywhere I will camp, except for overnight fishing trips possibly (where I'll need the 12V lighting). Besides, 12V light fixtures are flat and usually take up less space than 110V fixtures.
My only question was what type of setup did I want. Just a charger like Jack has or a converter. I finally decided on the same converter that Jack mentioned.
http://www.bestconverter.com/view_category.asp?cat=63 after I found out that the company replaced the single stage charger with a 3-stage charger. Although it appears to be rated to ouput 25 amps of 12VDC, the main polarity protection fuse is 30 amps, and it will handle up to 30 amps of 110VAC. I put in two 15 amp breakers, one each for my front and rear 110V outlets. It's all connected and working. Now, I can work on my interior in cool comfort with the A/C on.