I had a very similar problem and posted my solution in the Sunseeker forum under Generator Low Voltage:
While attempting to perform our monthly generator exercise under load on our 2017 3010DS, the generator started properly but did not trip the automatic transfer switch at the 30 second mark. We shut down the generator and restarted with same results. Shore power was disconnected both times. After reading several threads on the ATS, I went under the back bed and tested voltage on the ATS lugs. On shore power, it read 119V on both the shore leg and house leg. Disconnected shore power and all voltage readings were 0. Started generator, waited 30 seconds and tested voltage. Read 2.7V on generator lug, 0 on house feed. Puzzled, I checked breaker at generator and it was on. Cycled breaker off and on, same low voltage at ATS. Pulled generator breaker housing out and test voltage on the breaker - 123V on both sides of breaker. OK, figured problem was between generator and ATS. Traced power feed out of generator and into first storage compartment next to generator to check junction box. Junction takes aluminum stranded wires from generator and converts to solid copper wires to ATS. Inside box, I found my problem. Wire nut on white wire was melted - see picture. I cut both white wires, reconnected with shorter bare end on solid wire, added new wire nut, insulated tape covered by electrical tape. Made same repair to black wire in case. Ground wire was ok. Put junction box back together and made sure wire nuts were not wedged against the box. Turn on generator, watch ATS and whallah! Back in business. Ran AC for 1 hour to make sure. Replaced all covers on boxes.