Check the wiring on the sender, this will be on the top of the deisel tank and have a habit of corroding away meaning you may need a new sender unit but with a fixed body (not a tipper) you will have to remove the fuel tank, if the truck has side skirts then this makes tank removel difficult but not impossable.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lorries/trucks.