So yes, I will second this as a caution to use the RV NEMA 14-50 extension cords. Check out the failure I had doing this for a while with my Tesla. This happened just from the slight slop in the connector to the extension, and if there is any movement, a permanent connection may cause melting and possibly fire. I will say some of the J1772 extensions shown above where they connect via J1772 on each side may be a better cable.
Length of mobile charger
