Winnebago is generally one of the better regarded brands, though not a premium one.
With used units, and many times over with ones this old, the condition it's in and the care it has received are far more important than the brand. Look very carefully for signs of water damage: delamination on the sides (i.e. bubbled out siding), soft spots on the walls inside or out, water stains on the walls or ceilings, soft floors, etc. Pay particular attention to the cabover area, as this part has lots of seams and is subjected to some stresses and movement when driving. Some would say run away from any unit with an sign whatsoever of water damage; I wouldn't quite go that far, particularly in your price range, but do be very cautious and aware that usually any you find is more extensive than it first appears from the outside and can take a good bit of time and/or money to repair. Check that the sealants on the seams (particularly the roof) look to be maintained and in decent shape, not all cracked and dried out.
Have every system demonstrated for you: the fridge, water system including water heater, stove, oven, generator (assuming it has one), DC systems on shore power and on battery power, furnace, air conditioner, etc. If something isn't working, assuming it's something you care about, factor in the cost for repair or replacement.
Check the age on the tires; if they're older than about seven years, they probably should be replaced or at the very least professionally inspected (which, to do properly, requires dismounting them to check both the inside and outside). A set of six tires will probably run around $1000 for decent budget tires or perhaps half again more for name-brand tires.
The Ford chassis and engines are generally pretty reliable. The mileage would not be a concern to me in itself if the mechanics seemed to be in good shape. There may be some suspension parts that are due for replacement, either due to time or mileage or both. Check the ball joints and their seals and lubrication; if they have grease fittings (or ports you can put fittings in), that is a plus provided you keep up with greasing them. I wouldn't expect major engine (or transmission) work for awhile longer, though there are some non-major engine things that are not cheap sometimes. If it's a V10, replacing the spark plugs can be something of an ordeal due to tricky access. I assume probably the modular V8 is the same way.
There are a couple of recalls that might apply; if they haven't been done, it should be free at an appropriate service center. The first is a Ford recall for the cruise control brake pressure switch, which could short out and has been the cause of a few fires in parked vehicles. There are also some recalls for many of the RV fridges to help prevent fires; you can look that up with the fridge make, model number, and serial number.
Gas mileage I would guess is going to be somewhere in the vicinity of 7 to 8 mpg, likely closer to 7. If it's the odd diesel (presumably the 7.3, which I understand to be reliable but noisy and a little gutless) it ought to be a wee bit better.
