I can't imagine you having a problem even in the furthest north reaches of Florida without heating your tanks at all-Just keeping the interior of the trailer at 65 or above will allow enough heat to radiate through the trailers structure to keep your basement from freezing.
I've stretched my fall camping season very late many times in the northeast-Without even heating the trailer I've relied on my hot water heater to provide enough additional heat to keep the basement of my trailer a few degrees warmer on frigid nights.
It's gonna be 18 here tonight-yes I have winterized-But if I hadn't yet I wouldn't even worry about my tanks if I had 2 oil filled radiators set on high at 65 degrees in the fiver-they wouldn't shutoff all night but the basement would be in the upper 40s or 50s tmrw morning-I have monitored this with a remote thermostat(borderline ocd)
Remember that those tanks are big thermal storage units-it takes a long time to drop 50 gallons of water even a few degrees.