The Transit and E series are two different frame designs. The Transit (same as the Sprinter cab chassis) has a boxed frame that virtually has no flex. That's the reason we selected the Transit as our personal vehicle. It saved us overall height, cost and weight!
There is not much info out there on the frame flex with the E series. I would imagine very little flex as that platform has been used on countless RV's without any known issues other than the typical build quality garbage. In any case, I'm not saying you don't need to calculate for frame flex, I'm just saying it can't be much.
As far as the statement goes about zero frame flex on the Fuso.... I would be very careful! The fuso frame is extremely soft and flexes at the same rate as a Unimog. An empty cargo box can handle some of it, but cut some holes for windows and doors and you will see complications. Then there is the interior outfit that could suffer from the torsion.