I just traded away my 2004 Horizon 40ad, and here's some pros and cons on that coach;
It's an excellent floor plan, one slide in the bedroom is very nice, the side TV is the best layout IMO, plenty of basement storage.
Winnebago is still in business, so you are not buying an orphan. They have an excellent website which posts all the wiring and plumbing schematics. Their customer service over the phone has been very helpful to me over the years. Online parts are readily available thru a couple of dealers in iowa.
The basement air is less-than-adequate for any 40' model, so I D-I-Y added a rooftop air and the results were excellent. Adding a roof air as an adjuct to basement air
It is a good system, and in shorter coaches without full body paint is very adequate. Expect some major work to get the basement air working properly, it is basically two separate systems built in one unit. Once you remove the a/c from the coach D-I-Y, it can be serviced by any willing residential hvac tech. Lots of threads on this service.
All Winnies with basement air have ductwork running behind the rear cap up to and thru the ceiling. The ducts are made of foam board wrapped in foil, and are notorious for cracking at the seams. Climb under the right rear and with a flashlight look up alongside the ductwork, if split seams are down low they can be repaired D-I-Y. If split seams are found higher up, the repair requires removal of the rear cap($2000).
In 2004 the trans did NOT come with factory installed Transynd, so without records you must assume that the trans fluid needs to be changed over(2X) to Transynd($600).
Buying a m/h without service records means you need to start over with ALL maintenance to establish a baseline($3000). This means all fluids, filters, and belts need to be replaced, and probably an alignment too.
The original coolant tank on the engine(top rear, easily visible) was made of plastic and they ALL crack. It must be changed over to a metal tank, about $500 as I recall.
Expect to need all new TV/audio/Sat dish and wiring on any coach 10 years old.
Expect to need all new tires($4000+).
Check the operation of all slides, smooth, no binding, no gaps, no grinding. Check the operation of the jacks, smooth up and down, and easy to level. Slides and jacks are all operated by one HWH pump, which are excellent if well maintained.
Walk the roof to inspect for cracks, adequate caulking, sealing, etc. Inspect interior for any signs of water leakage, smoking, pets.
The RestEasy coach is comfortable to sit, but a real backbreaker to sleep on. Expect to replace it.
Standard engine was a Cat, very few were installed, most came with Cummins. In 2004 the Horizon/Vectra was optionally available with the 350 ISC(with exhaust brake, and 1050# torque), but in 2005 and later it became optionally available with the 400 ISL(with 2-stage engine brake, and 1200# torque)... I would much prefer the 400ISL engine, with the better engine brake and higher torque) in the 2005 and later models. If you never venture into the hills and mountains then not a factor.
You may find a 350 ISC with the Banks Power Pack, which was an excellent power upgrade, but it's no longer available, and no replacement circuit boards are available. Steer clear of this.
In 2006 thru 2008 they discontinued the 40ad model and replaced it with the 40kd, which is a very similar floor plan but they added a 4th slide. They also raised the ceiling about 6", which is an important advantage. In the 2004-5 40ad the Norcold(get rid of it) refrig is in the PS slide, and the slide box is about 1" too short to accommodate a Samsung 197 residential refrigerator. The additional slide height in the 2006 and later models make refrig replacement much easier. The front door is wide enough to bring in most any refrig with the refrig doors removed.
Depending on condition, added upgrades, and maintenance needed, I would expect to pay $80-90k tops, maybe a good bit less. And, I would allow at least $10k for fix-ups after the sale.
Mileage is not an issue per se, but the 22k coach only averaged 2200 miles/year, which is underuse. I would suspect the previous owner(s) never felt the need to do routine maintenance or replace tires as it was seldom driven.
There are plenty of 2004-5 Horizon/Vectra 40ad on the market, so don't accept the first one you see. It's an excellent model, at the right price you'll have a nice coach.