Steve, historically Newell did not make major changes in conjunction with model designations. As a matter of fact, most changes didn't even occur at a model year change. The change from the Classic body style (1980's) to the Model 2000 did occur at the beginning of model year 1990. EXCEPT, there was one Classic style coach built in 1993 (coach #317). Typically Newell changed model designations in the middle of model years. The body style designations are: Classic, 2000, 2000i, P2000i, 2020P, and the current p50.
For a sticky post that highlights model year, coach numbers, and new model designations see
http://newellgurus.com/showthread.php?tid=3457
Major visual and mechanical changes occurred at random times throughout the years. The new smiley face front cap (small round headlights vs the large rectangular Ford headlights) occurred in model year 2000, EXCEPT #538, the last 1999 had the new front cap. The transition from the Detroit Diesel 8V92 two stroke to the Detroit Series 60 occurred during the 1994 model year with #363 likely being the first with the Series 60 but there was a mixture of the two engines until #377 in the 1995 model year. 1993 model year saw some coaches with Aqua Hot units instead of the Primus (propane fired predecessor of the Aqua Hot) or the Raritan Marine Electric water heater and LP furnaces, #324 was one of the first Newells with Aqua Hot. Near the end of 1993 model year was the transition from 12.5 kw generators to 17.5 kw generators to support residential refrigerators. The generators were later upsized to 20 kw in 1994 but 17.5 kw remained in some coaches for many years. By the 1994 model year, some coaches had disc brakes on the front and/or the tag axles. 1995, with #388 Newell began adding surge-protection to all coaches. #398, a 1995 was the first 45'8" Newell. During the 1996 model year, some Newells were equipped with Dana Independent Front Suspensions rather than the Rockwell solid front axles, #416 may have been the first. #423, a 1996 was the first Newell with a rear bath and a mid-coach half bath. There were only 3 rear bath, mid half bath Newells built without slides. #432, a 1996 was the first Newell with the 'new' curved dash. #436 was the only 1996 Newell to have a slide. 1997 models saw expanded use of slide outs especially in the second half of the model year. Tires on Newells in the late 80's and early to mid 90's were 11R24.5 all around. With the addition of slides, Newell began installing 315/80R22.5's on the front. This was the standard tire size from July 1, 1995 until 9/30/1999. #443, a 1997, was the first Newell that can be upgraded to 365/70R22.5 tires on the front axle. Michelin Load Range L 315's are recommended by Newell on the front of the coaches with a max cruising speed of 65 mph. After the upgrade to 365/70R22.5, a max cruising speed of 75 mph is recommended. The 365's became standard after coach #1229, a 2008. There were a few coaches, around #450-458 thatcan't be upgraded due to stud spacing of 4" rather than 3".
Most of the coaches with the 8V92 had a N/S (rear to front) bed orientation. Those with the Series 60 predominately had an E/W (headboard to the side) bed orientation.
1999, coach #507 was the last Newell without a slide other than a corporate coach in 2010.
Let me know if you need information that includes more recent coaches.