I have door flaps but my tortoise is afraid of them, so if I want him to come out, I'll prop them up. That is when it really gets cold inside. If I leave them closed, he might make it out but he won't go back in, and I'm not always home when that happens.If its dropping to 75 when the thermostat is set to 85, that tells me that either you have too much air flow, or not enough heat.
If its getting cold with the door open, that tells me you don't have door flaps, or maybe your flaps are letting in too much cold air.
Just guessing here, but theres got to be a reason why mine are all more active and have more appetite in the same part of the world...
I have my heat sensor nearer the door so it is probably warmer further in. The heat sensor hangs near where his tail usually is when he is on the heat mat.
He came out today and ate quite a bit.