If there is slope away from the house, attach a garden hose to a bibb; open a few COLD taps in the house ( keep the hots closed) and open the bibb after shutting off the main valve. When working on a low section like you may be, you might even need to use a shop vac to either suck or blow the pipes down.
Sounds like you have a tempering valve on the toilet. These are used if you live in a cold climate. The water entering the toilet is so cold, warm moist air inside the bathroom condenses on the cold porcelain tank and drips onto the floor. By mixing the toilet supply to a moderate temperature, this problem is eliminated.
A tempering valve is a common plumbing supply item, but is not likely to be found at a box store. Any plumbing supply house will have it.