This should allow you put food in the fuel tank. Tested on TAC, should work with ioncross.

ModularFuelSystem_TAC.cfg

@TANK_DEFINITION[Default]
{
	TANK
	{
		name = Food
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Water
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Oxygen
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = CarbonDioxide
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Waste
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = WasteWater
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
}

@TANK_DEFINITION[Fuselage]
{
	TANK
	{
		name = Food
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Water
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Oxygen
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = CarbonDioxide
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Waste
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = WasteWater
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
}

@TANK_DEFINITION[Structural]
{
	TANK
	{
		name = Food
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Water
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Oxygen
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = CarbonDioxide
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Waste
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = WasteWater
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
}

@TANK_DEFINITION[Cryogenic]
{
	TANK
	{
		name = Food
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Water
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Oxygen
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = CarbonDioxide
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Waste
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = WasteWater
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
}

@TANK_DEFINITION[ServiceModule]
{
	TANK
	{
		name = Food
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Water
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Oxygen
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = CarbonDioxide
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = Waste
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
	TANK
	{
		name = WasteWater
		amount = 0.0
		maxAmount = 0.0
		utilization = 20.0
	}
}