Lowe's has a brass anti-siphon valve with half inch fittings. It's configured the same as the RV type, so it's just a matter of getting the two fittings to screw into the valve and installing it under your sink. While that's still no "absolute guarantee against any leakage", I've had this under my sink for 5 or 6 years with no issues. I do remove it every winter when I put the trailer in storage just to make sure there's no water left in the lines to "surprise me in the spring"... Some Lowe's stores carry it in stock, some only have it "special order. It's also available on Amazon for the same $17.
https://www.lowes.com/pd/Cash-Acme-B...E&gclsrc=aw.ds
https://www.amazon.com/Cash-Acme-245...66304581&psc=1