As someone else stated dual pain windows are a must. Not only will it keep the heat in, but it will reduce the condensation. I would also look for a heated floor for increased comfort. When your feet are cold, so are you. Also use plugs in your roof vents to help hold the heat in.
The above are found in higher end rigs, meaning more money. Since you have a budget to get those items mean you'll have to buyer older. I do not know of any gas rigs that have a heated floor meaning you'll be most likely buying an older DP.
Since you'll be buying older then I would also take into consideration the roof material. Rubber roofs can last only about 20 years before they become an issue or at least a concern.