Hi Sparkey Mark, I obviously can't speak for all the hotels in Beni but having just returned last week from the Flamingo Oasis, they certainly have a good heating system in the bedrooms, they are clearly marked with the +and- sign indicating the lowest and the highest temperature.
I personally don't like the heating on whilst I'm in bed, usually due to having had rather a lot of alcohol

so I altered it accordingly depending on the time of day, the room heated up very quickly on a morning, it seemed very efficient.
Although it was pretty nippy outside, especially at night, the public areas in the hotel were always warm too.
Hope this helps Mark, if you are cold, you can always order an extra blanket from reception