A 50' extension is probably unlikely due to concern with voltage drop. For longer runs I use a 30' 50a extension connected to a 30' 30a extension. I also have a 30' very heavy 20a extension that I have used a few times.
If you absolutely insist on no connections over the entire 50', creating your own extension is likely the only way. But for something that is not permanent, there would be no problem with connecting a 30a extension to a 50a extension. I would not be comfortable connecting a 50a extension to a 50a extension due to the concern with voltage drop.
