That's not too long of a trip. Your MTB will be fine. Getting a low end road bike is not as good an idea as it seems at first. You'll have to deal with the cheap wheels going out of true with all that camping gear weight. You might even have broken spokes. MTB are much more rugged than road bikes and can take alot more abuse (of all the type of cycling, touring is probably the most abusive on the bike). And shopping for a new bike is sorta stressful, not to mention that you need to break that bike in before going on your trip. You're better off getting slicks, cheap aero bars, and keep the money in your pocket to splurge on beers, burgers and bungalows. Certainly would be a more enjoyable trip.