It all depends on what you want to do with the RV. Most can be modified to work without running water easily, more work to adapt a water system.
Look for a unit with insulation in the walls as this cannot easily be improved upon and if possible thermal windows (large cost to upgrade)from there you can upgrade things like adding an insert in the roof vent, adding curtains to hang between the cab and camper / over camper door, etc.
Here are some things that I have done to make it more comfortable (I have a lot more to do to get it down to -40)
Class C mods