Depends what the active ingredient is. Some RV anti-freeze is alcohol based, some is propylene-glycol based. Most windshield washer fluid is alcohol based.

If you wait until the peak of the RV winterizing season, RV stuff can be expensive. I always watch for sales and the RV stuff ends up about the same as windshield fluid. Buy a few cases when the price is right so I have a few years to catch another sale