If you are buying bare cord to make your own, avoid any that have a "T" in the description. The "T" stands for thermoplastic, which gets stiff in cold weather.
More than you want to know about cord:
S types without the "J" are 600v Extra Hard Usage Service Grade.
SJ types are 300v Junior Hard Service Grade.
Code does not specify which cord type (S or SJ) is required for an RV, only length (minimum of 25'). Although SJ cords are lighter & less expensive, they are more easily damaged. Type S cords have thicker jackets, more fillers, and are much tougher (most could be used as a tow chain).
Either can be made with a thermoplastic or synthetic rubber jacket. Thermoplastic insulations are less expensive, but get very stiff when cold, while synthetic rubber are flexible warm or cold, but cost quite a bit more than thermoplastic. To add even more confusion, there are Thermoset cord types that can have good cold flexibility.
The "O" stands for oil resistant and can be added to either weight cord.
An additional "O" stands for additional oil resistance, a "W" for weather resistant. If there is a "T" in the cord description, it will be thermoplastic, however some thermoplastic cords leave out the "T".
Carol Wire produces both an S & SJ "Super Vu-Tron" rubber cord, however #10 is the largest size available on a standard order. This is an extremely flexible cord even below freezing.
Coleman Wire manufactures Seoprene 105 SEOOW & SJEOOW cord that is rated as extremely flexible at cold temperatures.
More information at LEX Products
and Coleman Wire
Lastly, most of the cord you can purchase at your local big box hardware store will be thermoplastic. You will need to go to a real electrical supplier to get the better quality cords, and they will not be inexpensive.