Personally I would look at 2000 and newer if you're considering one on a Ford chassis. The 1997 and some 1998 models still used the 460 cu. in. V8, and had a narrower front axle. Ford didn't make a 1998 model F53 chassis so the earlier versions of the 1998 motorhomes would be built on a 1997 chassis. The later ones would be built on a 1999 chassis.
The 1999 F53 chassis had a wider front end, but had a 275 hp V10, and the 4R100 transmission. 2000 through early 2005 models had the 310 hp version of the V10 and the 4R100 transmission.
Again Ford didn't make a 2005 model F53 chassis. So 2005 motorhomes built prior to March 1, 2005 would be built on the 2004 chassis. Those built after March 1 2005 would be built on the 2006 chassis.
The 2006 chassis was the introduction of the 3 valve per cylinder V10 which puts out 362 hp and 457 ft. lbs. of torque. It was also the introduction of the 5R110 5 speed transmission.