I do not know where you live, or what your weather is like, but if it is very hot/cold you may want to look into what they used to call an "Alaskan Pack" It has extra insulation, and is well worth the money over the long run.
Also ask them to show you how they are constructed, some are better than others. This all too is dependent on how long you plan on keeping it or using it in this manner...
My brother has a 5th wheel, he got for a steal, he lives in it 4-5 days a week. It has a fireplace, dishwasher, washer/dryer, big shower stall and a bath tub....(he says he got it for his "baby" to be comfy in while he is at work...baby is a 100+lb golden retriever, LOL) There are a lot of good buys out there right now, people just can't justify the expense with our economy the way it is.
Note, this is a home, and so you may be able to use it as a tax right off too, check with your tax preparer :D