Being mounted at the base of the windscreen may be preventing the GPS from acquiring & holding the sats. Can you mount the receiver on the top of your windscreen or on the hardtop if you have one. If the roof of the cabin is the only option then you may need to mount the mushroom on an extension long enough to give you a 360 deg field of reception.
Did you do a trial run on land to check if the unit was working OK? If all was good, would seem to be antenna related.