Answer:
Unfortunately, there is no MLS that covers the whole country. Some MLSes have joined together to form large state-wide MLS data feeds, but there is no national MLS at this point. We do, however, offer IDX service in 100s of areas, in nearly every state, and can combine MLS IDX feeds together into a single IDX plugin or mapping IDX search account. You can see all the states and areas where we offer MLS IDX feeds in our MLS Coverage section.