If you already had TF-FuelPump and then add Diesel Vehicles you have 2 options,

1. You delete the TF_FuelPumpConfig\FuelStations folder and let it regenerate the fuelstation files losing all your current fuel quantaties at each station and start fresh with all the values set in the TF-FuelPump and Diesel Vehicles configs.
   
   
2. You can leave the folder and files there and it will add the new line "CurrentDieselFuelAmount": to those files keeping your original quantities at each station, but the diesels quantity will be 0.0 instead of taking what is in the config like it would if you deleted the files, so you will then (with the server stopped) have to edit the amount for "CurrentDieselFuelAmount" to the desired quantity in each fuelstation file (this doesn't take long, probably 30 seconds).

Unfortunately these are the only 2 options i know of at the moment as i don't see a way of doing it automatically.

If you didnt already have it but you want to run the 2 mods together then just start the server once with both mods installed leaving it a few minutes to generate the configs. Stop the server,
edit the config in your serversprofile folder TF_FuelPumpConfig\config.json and the one in serversprofile folder Wilmas\WilmasDieselVehicles_config.json to how you want it.

If you have changed the InitAmount in the TF config or Diesel_InitAmount in my config you will have to delete the fuelstation folder from inside the TF_FuelPumpConfig folder to make the amount of fuel the pumps start with update to the new figure you put in. then start the server again and you will be ready to go.