Our diesel truck 2003 ram 2500 fires but not starting. Need help as travelling on 2003 Dodge Ram 2500

Please any advise. Our truck is having trouble starting. We changed air
And fuel filters and seemed ok but now not starting
Specially when it is hot. When we plugged it in
It seemed to help although it is summer. Why that.

Then even when cold it started but after longer cranking
Now we are travelling and it had to be towed. Waiting for
Shop to open to take it to. Anyone else have this prob????
It is diesel dodge ram 2500 2003. Any advise greatly

Asked by for the 2003 Dodge Ram 2500