Can I use any 50 amp extension cord?

If the cord does have the same connectors and wire size ("6/3", or 6 awg), it's equivalent to an "official" RV extension cord will work fine. There may be differences in the type of cable used (how resistant it is to abrasion or prolonged UV exposure, etc.) and the quality of the connectors, but I wouldn't worry too much about those things for your temporary use situation. In any case, it would be wise to keep the connections out of the weather if possible.

Do double check that you're getting the right connectors.

As long as it is wired correctly and wire is sized correctly any will work. You could buy wire and connectors and make your own if you wanted. And know what you are doing. The RV cables have a rugged outer sheathing for xtra protection being dragged around frequently.