Yes, if the Cradlepoint acts as a router, you could setup it's "WAN" address to be the next hop with the subnet being cradlepoint LAN subnet. Then hopefully (you'll need to test this), the route becomes active when the trucks comes in. And it'll have preference over the VPN route, so that Wi-Fi is preferred over LTE. You'll also have to setup static IP mapping so that the trucks' Cradlepoints each get their own static IP address everytime.
When two trucks have the same subnet on the LAN side, you'll have issues though...