Create a Query and include all the relevant fields.
Add an extra Field and put this in.
Days: DateDiff('d',[DeploymentFrom],[DeploymentTo])
Then use this in the Report with the new query as the Record Source.
=Sum(IIf([VehicleType]="H.Truck",[Days],0))
I tested this and it works.
PS
Could you place your Windows and Access versions in your Signature as there are some differences that we need
to know about from time to time.