At this time of global economic crises this is almost impossible to answer. Anything is only worth what somebody will pay and at present at auction some fleets are only making a small fraction of what they should be worth. However as a starting point, maybe if the trucks are well maintained, an optimistic value maybe made by devaluing as follows
20% year 1
20% year 2
15% every year after.

it is unlikely that in any market in the world that they will be worth more than that.

As said that I believe is optimistic and a s/hand dealer might offer a lot less.

hope this helps