Well, by now you probably have your tanks filled (considering your original post was yesterday). Hope you survived the night OK.
It is for this very reason I never open the propane valves on both tanks at the same time. My TT has 2 - 30 pound tanks. The regulator has the automatic switch over, which does work very nice. It also has a manual valve to flip from one tank to the other.
I learned, about 3 trailers ago, it is never a good thing to have both tanks open at the same time. You never know when one tank is actually empty, and it's too easy to run the second one empty and then you really are in trouble.
The first time this happened, I learned my lesson real fast. Every since, I keep one tank completely closed and only one open at a time. When propane run out, we will know immediately. At that point, switch over to the the other tank, and immediately get the empty one filled (very, very first possible chance). We've never run out completely of propane doing it this way.
Currently, in Indiana, temperature is around 20 degrees. It's been hovering between 35 and 20 for quite a while now. We use our camper all the time, even though it's parked at home. I'm running through one 30 pound tank about 6 days now. My camper is 35 feet long with 3 slides, and we supplement with 2 electric ceramic heaters too.
When temps get a bit colder, we'll be moving into the house. When fill ups reach 4-5 days apart, it's too costly.... time to move into the house.