it depends upon what you want..
I've road tripped in everything from a VW Campervan to a Mercedes 2-door Diesel Coupe to a Lexus GS300 to a (current) Toyota Sequoia.
It depends upon what you need, and what you want. A sportier car with lots of Vrrrooomm is a really cool car to roadtrip in, but is sometimes limited by what you can stuff inside. When I was alone or travelling with just one other person, a 2 door worked fine -- although I found we typically had to stuff stuff into the back seat as well on camping trips. And you may be restricted from taking those bumpy dirt roads you find every so often.
Now, I use the Toyota Sequoia SUV -- One of the mid-to-large SUVs, but with reasonable mileage (20 mpg on the highways), and with enough room to put camping gear for 4-6 in the back, plus seats for at least 5. It's also got some off road capabilities, so I feel more comfortable on unpaved roads than my low-slung cars. Pretty comfortable -- with enough room no one feels squashed -- at least until you get more than 5 inside. And in a pinch I can sleep in the back (never have, though...) Not quite as much Vrrrooommm though...