Before you start getting worried, this does not need to be a single jump that lasts for 30 seconds. This is a cumlative trophy that counts for every single jump you make in a vehicle. Hit all of the ramps you can find and this will unlock eventually. It should take 40 or so average ramps in the truck before you get the 30 seconds needed to unlock the trophy.
Get 30 seconds of total airtime with the vehicle