It should be on the wall by the driver's seat. Look under the dash. Tiffin also places a build sheet in the closet.
If it's not there ask the question on the Tiffin RV Network and I'm sure one of the members will know for sure where they were placed in 1998.
Tiffin RV Network
Jon & Sue Francis (Retired U.S.A.F.)
Lil Girl-Rescued, Abby Rescued, Peaches Rescued
06 Allegro 35TSA Workhorse Chassis
2013 Chevy Spark Dinghy