A belated ; Welcome to iRV2.
There have been posts about the transmissions being programed as 5 spds, rather than 6. Depending on the transmission model . 2000 series? 2500?
Main reason is engine performance, in relation to vehicle weight. 6th gear is a double overdrive, and to avoid dropping the engine out of it's power band, 6th is not available. No sense in having the trans shift into 6th. if it won't stay there, on even level ground.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.