Actually I would expect it to be in worse shape if it was used as a true brush truck.
The choice of tires makes me think they may have been.
Our department used deuce and a half's for brush trucks. They spent more miles in the woods/fields than on the road.
We used city water in the tanks, but the addition of Class A foam wreaked havoc on the beds and frames.
They were heavily used during our brush fire seasons and we spent many late night/early mornings rinsing them down after the fun was through.
They spent their entire lives outside.