Hmmm - a 1999 F53 chassis with a tag axle? Better check your date of manufacture for the chassis, it is often different than the model year of the motorhome itself. I thought that 1997 (chassis year) was the last year that a tag axle was used with the Ford chassis. I could definitely be wrong, but please double check your build date. If you actually have the earlier 1997 Ford chassis then you'll have the 460 V8 engine instead of the newer V10, so it is an easy deduction there.
Unfortunately, the Koni web site does not list a tag axle shock for either chassis. Keep in mind that that the tag axle was NOT installed by Ford but by the coach builder - National for your Dolphin. Your tag axle was probably made by Dexter and packaged with Mor-Ryde suspension.
Please confirm your chassis and we can help.