I saw something yesterday about the right size and color as you describe, moving slowly through some pine forest near a stretch of I-80. It certainly wasn't a moose, so it does seem likely it was a thermarest instead. It looked a little deflated and as if it had seen some wear, but not as much as you might see on a chunk of discarded styrofoam that had been blowing out there in the wind for a few years.
Ah, excellent; I may have to hunt down the rogue thermarest.
I saw one the last two nights, except it wasn't really green....