Use the stuff made for boats. Its made with propelyne glycol. The cheaper stuff made with alcohol can damage rubber O rings and impellers. You can also get a higher freeze point of -100 . West Marine delivers it if you order online. You also should use diesel stabilizer in the gas tank and oil stabilizer in your crankcase. WestMarine has a video online that tells you how to winterize your boat.
You could probably prevent an engine from freezing with RV antifreeze, but I definitely WOULD NOT run an engine with RV antifreeze in it. It has a different boiling point and different chemical composition which can cause your engine to overheat or deteriorate seals, etc.
Actual engine antifreeze (ethylene glycol) is formulated for your engine-- just use it instead-- it costs a little more, but you won't have to drain it out when you get ready to use your engine again. BTW, it IS very toxic, but has a sweet taste so animals are drawn to it... drinking it, even in small quantities, will cause kidney failure and an agonizing death-- clean up any spills, don't just drain it out on the ground when emptying your cooling system.
Another tip--make sure you dilute the antifreeze with water per the container directions. Straight antifreeze WILL NOT increase your protection. You can buy also buy it pre-mixed 50-50 with water, so be sure not to dilute the pre-mixed stuff.
If you want to use RV antifreeze in a marine engine, I'd check the engine manufacturer's recommendations. If you don't have the owner's manual, just look it up online.