My 2004 Tahoe with a 5.3 Liter engine has a coolant leak that my mechanic and I have not found. The first sign of the problem was the rear heater would not blow out warm air, we added coolant and the system began working. We have checked for external leaks and have not found any. It seems to have leaked almost a gallon over the last 3 months, but the oil level was not too high and there was no evidence of the oil being milky. However, the oil does tend to get very nasty after about 1500 miles. I bought this vehicle used with 55,000 miles on it, and my mechanic says that the previous owner may have never changed the oil and this would cause the oil to become nasty pretty quickly, but if we continue to change it frequently it would clear up. My question is, where could the antifreeze be going, and how dangerous is it to drive (we are getting ready to go on vacation, 1200 miles round trip)?