Mysterious coolant leak

About 18-20 months ago “coolant low” light came on. I filled reservoir with coolant, did not come back on for 4 or 5 months. It has progressed over time until now I am filling the reservoir at least once a week (or about 200 miles). It takes about 1/2 gallon to fill this every time. I have had the jeep to the dealer about 6 times in the last 5 months. They have determined the pressure in the system is fine. They find no evidence of leaking coolant in any of the normal places. They replaced the radiator cap. I had the oil changed right before Christmas and there was no coolant in the oil. There is no coolant in the exhaust. THere has never been a puddle of coolant under the car. Any one have any suggestions? It is a 2001 jeep grand cherokee laredo. I really like this car and hope to get this fixed. It has about 135,000 miles on it. Thanks

Ask friends and associates for the name of a good independent shop and take the vehicle there. It should be pressure tested.