In 1.54 RC, "isNumber" returns false for the following config entries in CfgVehicles:
B_Slingload_01_Ammo_F/transportAmmo
B_Slingload_01_Fuel_F/transportFuel
B_Slingload_01_Repair_F/transportRepair
B_Truck_01_Repair_F/transportRepair
B_Truck_01_ammo_F/transportAmmo
B_Truck_01_fuel_F/transportFuel
Truck_02_box_base_F/transportRepair
Truck_02_Ammo_base_F/transportAmmo
Truck_02_fuel_base_F/transportFuel
C_Van_01_fuel_F/transportFuel
I_G_Van_01_fuel_F/transportFuel
Truck_02_fuel_base_F is the parent of O_Truck_02_fuel_F that commy2 tried with.
Using getNumber, the transportFuel value of Truck_02_fuel_base_F is reported as 1e+012.