What you are looking for (and so am I) is an "expedition" touring bike. If you lived in Europe you'd have a plethera to choose from . Unfortunately for us in the US, there are practically no inexpensive solutions. REI (Recreational Equipment Inc) makes a pseudo-expedition bike for $850 - the Novara Safari. It will do what you need it to. Aside from that, it's either custom - Bruce Gordon Cycles, Sakkit Cycles, etc. or jury-rig a mountain bike.
The key elements that separate an expedition touring bike from a mountain bike are:
Geometry - chainstay length should be sufficiently long to allow your foot to clear your panniers without shifting the center of that load much past the rear axle. If you've ever descended a steep dirt road with 50 lbs of gear you'll know why this is important. 17.5 inches is a minimum. Additionally, a long wheelbase contributes to stability.
Braze-ons - a true touring bike should have braze-ons for front and rear racks, 3 water bottles, and fenders.
Finding a touring bike with all these features and 700c wheels is easy enough. The trick is to find them on a bike with 26" wheels. 700c wheels with fat tires are fine for well maintained dirt roads or a little rougher in flat country but I live in Arizona and my Trek 520 with 37mm tires can't cut it on dirt mountain roads with a load. I love my 520, it's perfect for pavement - flat or mountain with a load.
Good luck. I wish I had summers to tour. Time and money - neither is much good without the other.