I am assuming the boat is in the water. Is your engine fresh water cooled or raw water cooled. If FWC you are probably fine, especially if the water has only a thin crust of ice or no ice on it. The water temp is high enough to prevent freezing of through hulls and the antifreeze in the FWC system will protect your engine block.
If raw water cooled then it is probably not needed for only one cold night but you can run antifreeze (pink stuff) through your engine. The easiest way to do this is to fill a 5 gal bucket with 3-4 gallons of antifreeze, close you water intake, remove the hose from the valve, place the hose in the valve and start the engine. Run the engine until you see pink coming out of your exhaust.
I would look at your fresh water system. Empty it out as much as you can. Then fill the tank with a few handles of cheap vodka. Run/pump your faucets until you smell the vodka. The reason I use vodka is the pink antifreeze imparts a nasty taste to the fresh water lines and tank. The vodka is much more pleasant.
Finally I would pump out your holding tank, close your head through hull, remove the hose and put the end into an antifreeze bottle or bucket of antifreeze, and pump the head until you see pink in the bowl. Then empty the bowl into the holding tank.
It takes me only about 4 hours to winterize my entire boat.
Ocean View Condos