It's called a 12-inch "rough in". That's going to give you the widest range of toilets. Most will have some distance between the back of the tank and the wall, and thus would fit on 11.75" or whatever. With the full 12", the world is your oyster.
This assumes, of course, that your mobile home uses a standard-design toilet, which it sounds like it does. You would know this better than I.
But note that while you COULD buy any comfort height toilet from Lowes, HD, etc., you don't WANT to, because many of them suck at flushing. I can tell you that HD has a $99 elongated comfort height toilet that works very well and is surprisingly well-made, the Glacier Bay. Also, have a look at Terry's toilet review section and the articles on here.