Two answers depending on the outlets present on the generator.
IF the outlet is a standard 15/20 amp Duplex recpical you have multiple choices
1: Dogbone with 50 amp outlet and 15 amp plug (I have one)
2: Dogbone with 50 amp outlet and 30 amp plug (You should have one of these anyway) and 2nd 30 to 15 -OR-- A device called a power maximizer that is a 30 amp outlet designed to plug into BOT halves of a 15 amp duplex This is sold at Geneator stores as a general rule, NOTE: SoME generators, may not like this device, but frankly, I've never seen one that objected to it. IF the geneator is 120 volt ONLY it will not object to it.
IF the generator has a twist lock plug.. The proper adapter for it (Ask at the generator store) to either 30 or 50 amp RV, if you can only adapt to 30, then the standard DOGBONE to your fifty amp rig.
In any case this is all off-the shelf (or wall depending on packaging) hardware, Nothing special, Nothing you need home brew.