Mysterious fill - gray water tank

Is there any way for water to go in to the gray water tank except by the drain lines? In other words, how can my gray water tank get full when water is rarely used?

We spent 10 days at Indian Point close by Silver Dollar City and rarely turned the water on. Some days not at all. We ate breakfast only and microwaved waffles using paper plates and cups so there was no dish water used. We left the TM 23 every morning by 8:30, spent the day at the Southern Gospel Picnic eating lunch and supper at the park and didn't return until 7 or 8 PM. We watched a movie or two and went to bed.

OK, I emptied the black water tank once during our stay into a tote tank. Only had about 4 gallons in a 15 gallon tote. At the end of our stay I pulled the toilet valve and the gauge was showing less than half full. I didn't need to tote tank then and only needed to drive to the dump station. When we cranked up the front end to hitch up, the gray water tank started leaking out the top vent hole. It was full to the top. I could understand this if we had done dishes, brushed our teeth and shaved, etc in the TM but we did not. Mainly because we are full-grown and the TM powder room is tiny.

Am I missing something?