Anne, I bring in the "Can't be replaced plants " also and put them in the bathtub of the extra bathroom. We get these crazy freezes so rarely that I haven't bothered with an alarm. I'd probably still be up and down all night anyway.
The water jugs keep the outer reaches of the greenhouse warm.
Shady - Sounds like you need to beef up the openings on your greenhouse. I found some outdoor repair tape, and I used it to tape bubble wrap on a few spots that were hard to weather seal. You might try taping the bubble wrap over the opening to slow the cold air seeping in.
Elaine, I'd be putting a blanket over your little greenhouse for good measure !
