40 to 50 Volts on Generator (Neutral to Earth) "Normal"?
It turns out that the OP's model of generator is peculiar -- its 120V is set up in the UK jobsite fashion (i.e. two 60V hot legs and a neutral), which when combined with the way the 120/240V switching works on the 120/240V models of this generator, yields a neutral at an odd spot.

The only fix is to use a 240/240 isolation transformer to establish a new neutral point, or to rewire the generator (which'd void its NRTL listing).