Where did the glycol go?

We have had our TAB CS-S trailer for about a year and a half. On our first trip with our TAB we used the heater and the heater worked very well. It actually got too hot in the trailer. We didn't need the heater again until early this year and it did not work well. Camping trips following that one did not require the heater and so I forgot about it. We needed the heater on a recent camping trip and it reminded me of the one trip earlier in the year where the heater did not work right. I researched on the forum and the answer was to check the glycol level. Our glycol reservoir is behind the toilet and sure enough it was low. I could see the glycol level was at the neck of the reservoir. I added the recommended glycol to just above the minimum level. On our last trip I made sure to test the heater. It worked for a while and increased the temp about 5 degrees. I checked the glycol level when we got home and now I do not even see the glycol in the reservoir. Where did the glycol go? I am going to fill it one more time and test it.