It's not the brand of cord you buy, it's the code designation it has on the jacket that makes a difference. RV shore power cords normally have STW designated jackets which is thermoset PVC. Will always get stiffer when colder. Our Marinco and Conntek cords have the same jacket specs.
If you want a flexible cord, you want one that is elastomer based like SOOW, but you'll have to buy the cable and install your own connector and plug. I thought someone makes a flexible RV cord, but can't find a link at the moment.