It is "low" toxicity, and can be safely used in all water systems--plastic and metal, but will stain acetate (I doubt that is used in water systems).
To determine how much you need in 10 feet of 1 1/2" pipe, let's just do some basic geometry to determine the volume of the pipe.
The formula for volume in a cylinder (your pipe) is:
Radius (squared) X pi (3.14) x length. Use like units (inches with inches, feet with feet, etc.). The radius is 1/2 of the diameter.
So find the area of the Pipe Inside Diameter by taking 1/2 of the nominal diameter (1 1/2 inch pipe is actually a little larger than 1 1/2 inches), which is 3/4" x 3/4" (use decimal-- .75" x .75") = .5625.
Multiply .5625 x 3.14 =1.766, which is the area of the pipe in square inches. Then, multiply 1.766 x 120 inches (10 foot length converted to inches) = 211.9 cubic inches. A gallon of liquid equals 231 cubic inches, so a gallon of RV antifreeze will fill up your pipe.