Rhino Iguanas hail from Hispaniola... So it's a good idea to keep an eye on the weather there to help you get an idea of what your pet needs
http://www.weather.com/weather/today/Gonaives+Haiti+HAXX0001
It's the coldest part of the year (in the northern hemisphere) right now and the overnight lows there are still in the low to mid 70s )at least tonight). I would watch that page to give yourself target temperatuers...
^^Great post/advice...
I've never kept a Rhino Iguana, but I have an adult Argentinian Tegu and he (quite naturally) displays a wide range of activity, food intake, etc from summer to winter. I suspect such things are very normal to a cold blooded creature.
A practical response to your question though, consider connecting the heat emitter to a thermostat and let it be on whenever the temp drops below XX*F and let your research determine what XX is.