It surprises me how heavy PVCs are. The 6.4m A Class Carthago Compactline has a MIRO of 2795kg, which seems to be less than your 5.4m PVC. I guess it is in the lightweight construction used but there is nothing flimsy about the Carthago finish.That could well be the case especially with the larger coachbuilts and A Class MHs where the 3500 Kg MAM is always going to be very tight. I understand that some manufacturers have been reducing the weight of the furniture but there is only so much that can be done before it becomes too flimsy.
I have seen figures showing that the MIRO difference between a 5.4 m Ducato PVC and 6.4 m PVC is an additional 280 Kg. Presumably due to all that extra steel body and more furniture. On paper, very popular 6.4 m PVCs already have quite tight payloads. Add lots of bells and whistles, plus 4 occupants, the available payload becomes inadequate so 6.4 m vans are more likely to be overloaded unless a lot of care is taken. 5.99 m Ducato PVCs are the sweet spot for 3500 Kg.