Now, according to the MotoMama Spec whatever is fed into Vin should be available at the Motor outputs
No, that's not the case. The L298 chip, which is what a MotoMama uses, always loses 2V minimum, going to 3V at 1A and 5V at 2A... check the L298 datasheet.
But that said, if your'e putting 20V in you should get at least 15V out, 18V under no load.
Suggest you post a schematic or a Fritzing of what you've got hooked up.
however using the "analogWrite()" command i get not more than 4.8 V on the outputs.
.... not sure what you mean here. Are you measuring voltage with the analogRead() (did you mean?) in which case you shouldn't be putting more than 5V in the first place.