Go to an electrical wholesaler and ask for 3C/10 STW cable 600V. (Two conductors are hot + neutral and the 3rd is for ground) You should be able to buy it by the foot. The Marinco cord we have is STW jacketted. If they have 300V, that's all you need. I think the spec for the cable that normally comes with an RV is "SJOW" jacketted IIRC. That should be a bit cheaper if they have it. The STW jacketted cable is more flexible in cold temps. You need a cable jacket that has a "W" rating for wet locations.
If you can replace 15 amp cord ends on a regular extension cord, you will have no trouble with the 30A ends.
Crazy thing with the cordsets that are out there (Marinco, Conntek, etc.) is that buying just a 30 amp plug and 30 amp connector can cost almost as much as a complete cordset. I'd see what's on ebay or Amazon. I found the best price on a 30A connector at Overton's Marine.
Ditto on the voltage drop concern.
Gil & Deb & Dougal the Springer Spaniel