1998 Chevy Tahoe losing coolant

I have a 1998 Chevrolet Tahoe with 210,000 miles on it. I went out to get gas the other day, the gas station is about 3 minutes from my home, when I came out of the station I noticed my temprature gauge was reading about 240-50 when it normally runs about 200-210, I pulled ahead and parked it and shut it down, I checked the coolant level and it appeared to be completely gone, I let it cool off and topped it off, I was able to make it another whole day before this same problem accured, I replaced the thermostat and that wasn't the problem, I then checked the coolant level again and it seemed just a little low this time, I also checked the oil and it was way overfull and milky when all the previous times it was level and clean. I see no obvious leaks under the truck or in the engine compartment so I wonder what the problem may be? Where could it be going and what could be causing it to run out when there are no leaks? Someone said intake gaskets, if it is and I replace them, and change the oil will the engine most likely be ok?