No, an overflow reservoir is not part of a normal radiator replacement. But, you can check your invoice to verify.
I agree that Caterpillar ELC is a better coolant (as are most of the new generation OAT-based coolants). It is maintenance free for 6 years vs annual checking and replenishing the SCA in "regular diesel coolant".
Also agree you are looking for a coolant leak from cooling system to ground if no "frothy chocolate mouse color" to the oil. That was way too much fluid loss to not be noticeable if it went into the engine or transmission oil.