Well... First. You can not always "Just drive the class A" There are places I go where parking is not that easy with 40 feet of class A plus cargo carrier.
Second, the class A I drive peaks at 11 mpg Freeway, about 4 city.
Now the car I tow cost me about 1 MPG on the MH. but.. When I go to the store for Bananas, as I did on my way home from Church, Sunday. I'm looking at 20-30 MPG depending on the road.
Second. if I rent a car, turns out that my insurance company says the motor home is not an automobile so they won't cover the rental.. This triples the cost of the rental.
That said.. There is a definite advantage of driving the house to the grocery store... I buy ICE CREAM.. it does not melt before I get home (Home being the parking lot of the store).
However one of the goodies I bought for the car.. A cooler, So the Ice Cream stays cold on the drive back to the RV park.