I think some boas do go off feed during the winter. My colubrids are the same way - they don't stop eating, but they definitely start skipping meals here and there. Even my garbage gut female MBK refused last week, where normally she's the type to tag fingers unless you use a hook to get her out of her enclosure.

IIRC even if you keep their temps and humidity up, they can still sense other seasonal changes, like barometric pressure, and that causes them to back off of feeding. In the spring my colubrids also get much much "foodier".