The American Heritage is the top of the line for American Coach. They are truly beautiful coaches. The Cummins 500hp ISM is a great motorhome engine. It produces 1550 ft lbs of torque from a 2070 lb engine. By comparison, the Cummins ISX and Cat C15 weigh 3100 lbs, over 1000 lbs more than the ISM. Sadly, Cummins discontinued the ISM in 2008 for emissions.
There are two types of tag axles used in motorhomes. The bus type will have a positive offset style wheel similar to the front wheel. The other type has a negative offset wheel (deep dish) similar to the outer rear axle drive wheel. Monaco uses the dish style and Country Coach and American Heritage use the bus style. They are both great, but the dish style lifts automatically at low speeds in tight turns or backing. The trick is to adjust the air pressure in the tag axle air bags just right to prevent the front end from bobbing.
Make sure you thoroughly research pricing on both online web sites and NADA bluebook. Current motorhome pricing is all over the place. Also, check all the systems in the coach. Look to see if the Allison transmission has been converted to Transynd synthetic fluid. This is a very expensive conversion, especially in the 4000 series transmission. Check dates on all the tires and batteries.
Best of luck!