Dunno, I have a newer, 2016, model and no flexing that can tell.
The idea being that there's more surface support having it on the bed. I have a flat bed. I doubt there should be any flexing if it's level...
I use the little green bubble things to guide me on leveling, but it's "Grab a beer, sit back and watch..." thing heheh...
I would be concerned if it's flexing, something's not level or being supported as it should be...
One way is to "Jack it up" to level and then then lower "ALL jacks" till they start to show out of balance...
It may be, one corner is not quite right...
Hope that helps..