You are working on a project to develop a Java program for a car rental company. The program should have the following components:

An abstract class Vehicle with properties such as make, model, year, and an abstract method calculateRentalCost$($int numDays$).$

Concrete classes Car and Truck that extend the Vehicle class and implement the calculateRentalCost$($int numDays$)$ method based on their respective rental rates.

A RentalManager class that manages the rental process, including methods to add vehicles to the rental fleet, rent a vehicle to a customer, and calculate the total rental cost based on the number of days and the type of vehicle.

Implement the necessary classes and methods to create a functional car rental system. Demonstrate the usage of the system by creating sample vehicles, renting them to customers, and calculating the rental costs based on the number of days and the type of vehicle. Utilize inheritance and polymorphism concepts in your implementation.