For a used motorhome, particularly one that's fifteen years old, condition is arguably the most important consideration (well, besides whether the layout and design would work for you). I'd prefer a lower-end brand and model in excellent shape with no water damage over a high-end unit that has been neglected. Mileage is of little importance in itself, though of course a unit with especially high mileage may have a lot of wear and tear in general (though the opposite isn't necessarily true--there could be a lot of use and living without a lot of miles traveled). A motorhome drivetrain is very seldom worn out through use; the house part tends to wear out much sooner, or the drivetrain suffers problems due to improper or insufficient maintenance or to accident.
In terms of the layout and such, I would suggest thinking about renting a motorhome for a few days before buying. You'll probably find a lot of things to look for (or to look to not have), and generally have a better idea of what to expect. In any case, whether or not you rent, spend some time in any motorhome you're looking at to mentally, if not physically, go through the details of your activities in it. See how the counter space and included appliances might work for making a meal. Sit down to watch TV (if you'll be doing that), and see if it's comfortable and if the view is adequate--or figure where you migh put the TV if it is not. Stand and move around in the shower, the bathroom, etc. See how easy or hard it is to make the bed. Figure out where you might put the trash can, the dirty clothes bag or basket, the damp towels, and so forth.
