Yeah, a full stretch out doesn't really work in my van, either, but I can deal with it for a few hours. I can see where a roadster would be absolutely impossible.
At least your getting good gas mileage. You can use the savings to pay for the room.
By the way, when I do stay in a motel, I always stay in cheap places like Motel 6 and Super 8. They've renovated most of them, and while they aren't fancy, they're clean, the beds are comfortable, the shower works, and they have a TV. They usually have a fridge and microwave, but I seldom use them. You can usually find one under $75, and I recently found one for $37!