There have been eight instances where Dallas was the only NFL game being played at that time, and in most other game they have been broadcast to nearly the entire nation.
Week 1, TNF at Philadelphia, 100% broadcast coverage
Week 2, 1:00 vs NYG, about 50% coverage, from Nevada to Maine, Wyoming to Florida
Week 3, 4:30 at Chicago, about 90% coverage, televised in all but four states
Week 4, SNF vs Green Bay, 100% coverage
Week 5, 1:00 vs NYJ, over 90% coverage, televised everywhere except Northern Cal, Nevada, Indiana, NC, SC and Miami
Week 6, 1:00 at Carolina, broadcast to about 90% of the US
Week 7, 4:30 vs Washington, broadcast to all but the great lakes region and northern plains states
Week 8, 4:30 at Denver, broadcast to about 90% of the US
Week 9, MNF vs Arizona, 100% coverage
Week 10 - bye
Week 11, MNF vs Raiders, 100% coverage
Week 12, 4:30 vs Philadelphia, broadcast everywhere except Georgia, Mississippi and Louisiana
Week 13, Thanksgiving vs KC, 100% coverage
Week 14, TNF at Detroit, 100% coverage
Week 15, SNF vs Vikings, 100% coverage
Week 16, 1:00 vs Chargers, about 75% coverage
Week 17, Christmas Day vs Washington, 100% coverage
Week 18, Dallas at Giants: there is zero reason for this game to be broadcast anywhere other than those two team's home markets - but I'll believe it when I see it.