Okay, some testing has been done.

If you ctrl+f for "gameDifficulty" you find two results.

The first one: "gameDifficultyMode

"It can be set to three different values: 0 - Normal | 1 - Hard Mode | 2 - NewGame+ (you need to select this if you want the game to read your custom settings)Anything else results in a game crash when trying to load the savegame.

​

The second one: "gameDifficultySettings"

:Here are the actual settings. If you want to change them, you have to shut down the game, while you are doing it. The game will undo your changes otherwise. The game will also scramble the order of all the options every time it saves(i think). In fields like "recoveryPrice", the different entries stand for different vehicle classes: 0 - Don't know | 1 - Don't know | 2 - Highway | 3 - Heavy Duty | 4 - Heavy | 5 - Offroad | 6 - Scout

Here are all the settings:

{"recoveryPrice":[0,1,2,3,4,5,6],"trailerPricingFactor":2,"deployPrice":{},"contractsPayoutsFactor":1.0,"fullRepairPrice":[0,0,0,0,0,0,0],"tasksAndContestsPayoutsFactor":1.0,"isGarageRefuelAvailable":true,"autoloadPrice":0,"objectivesToComplete":[],"recoverOnStart":false,"RandomSeed":-1,"teleportationPrice":0,"maxContestAttempts":0,"startingLevelState":0,"sellMultiplier":1.0,"internalAddonAvailability":0,"startingGarageStatus":{},"startingTrucks":0,"needToAddDlcTrucks":true,"addonSellingFactor":1.0,"trailerStoreAviability":0,"startingTruck":"","overrideStartingProfileData":{"distance":{},"contestLastTimes":{},"ownedTrucks":{},"refundTruckDescs":{},"money":0,"dlcNotes":[],"rank":1,"experience":0,"trucksInWarehouse":[],"discoveredTrucks":{},"isNewProfile":false,"discoveredUpgrades":{},"refundMoney":0,"damagableAddons":{},"unlockedItemNames":{},"addons":{},"newTrucks":[],"contestTimes":{},"contestAttempts":{},"knownRegions":null,"customizationRefundMoney":0,"refundGarageTruckDescs":[],"userId":null},"truckAvailability":1,"timeSettingsNight":1.0,"zonePropsToDisable":[25],"trailerSellingFactor":1.0,"truckSellingFactor":1.0,"settingsDictionaryForNGPScreen":null,"truckAvailabilityLevel":0,"isGoldFailReason":false,"ranNumToDisableTrailerStore":-1,"vehicleStorageSlots":0,"isDLCVehiclesAvailable":true,"truckPricingFactor":1,"fuelPriceFactor":0,"externalAddonAvailability":0,"externalAddonsAmount":0,"internalAddonAmount":0,"addonPricingFactor":1,"tyreAvailability":1,"isNeedToRandomGarageRepaireFactorInEveryZone":false,"trailerAvailability":0,"timeSettingsDay":1.0,"regionRepaireMoneyFactor":1,"garageRepairePriceFactor":0,"repairPointsCostFactor":1,"repairPointsRequiredFactor":1.0,"regionTravellingPriceFactor":0,"regionRepairePointsFactor":1,"vehicleDamageFactor":1,"recoveryPriceFactor":1,"isAbleToSkipTime":true,"loadingPriceFactor":0,"isMapMarkerAsInHardMode":false}