The problem in both trailers was that the axles weren't located far enough back on the trailer for the weight
distribution... the necessary 15% tongue weight
. So in the 21' the solution was to add tongue weight
... the 300 lb slug. IIRC, the fresh water tank on the 17.5 was located in the rear and as it was filled, it unloaded the tongue weight causing instability. The fix was just not to carry more than about 10 gals of fresh water when on the road.
The extended tongue of the new 17.5 would, in fact, alleviate that issue as it would inherently place more of the trailer's weight on the tongue. The only issue with it that I can see is that you now have a 14' trailer on a 20' frame...
Both the 17.5 and 21' are fine trailers, but the 21' is only about 400lbs lighter than the 25' and the 25' has significantly more room with a full-sized queen walk-around bed. The 25 is no more difficult to maneuver than the 21... I just never understood the market appeal of the 21.