I have a 1994 RV that has a chevy 6.5 turbo diesel engine and a 4l80e transmission. For some reason my coolan reservoir tank is bubbling. I was told it was a leaking head gasket and to confirm to do a combustion leak test. So i did the test and it was changing colors. I did a blue devil head gasket sealer and the test is coming back normal now (no change in color). But the tank still bubbling. We have replaced the thermostat, the radiator cap, and still nothing. What can it be?