The temps you have are fine. Maintaing the same temps as the room ambient temp falls is a balancing act.

There are a number of issues to grapple with. Shape and how it is vented, (is it tall open top or low and side vented for example) How variable the ambient temp is does it drop and stay or bounce all over the place?

Some ideas are to insulate the enclosure so there is lower heat loss. and a bit of a buffer against the cool room temps, you still need to move fresh air in and out too. The other ideas in conjunction is to alter the configuration of the enclosure to something that is side vented if it is not already that will help hold heat in. I have a number of styles of enclosure side turned tanks regular tanks custom made and reptile designed as well as a rack each is unique in the methods used to over come the same base problem you are experiencing. There is no one answer and because something works for me does not mean it will for you.

What kind of enclosure is it? Maybe we can come up with some ideas that might work for you.