I sent an email to CruiserRV asking why the difference in tongue weights on the two 189s (FBS and FDS). Here is their reply:
Sent: Thursday, March 06, 2014 10:18 AM
Subject: RE: 189FDS vs 189FBS
The 189FDS has a larger slide and is placed a little different over the axles than the 189FBS.
2018 Lance 1475
2013 Nissan Frontier, aka Pretty Red Truck