Well very frustrated. So I will vent a bit. We had a travel trailer with 30 amp 120 service, had and still have my Honda EU 3000 IS and it did a great job handling the travel trailer with the transfer...
Your electrician is wrong. Camping World is correct.
NC (normally closed) input goes to your cord for straight through 50a 240v power.
NO (normally open) input will have L1 & L2 bonded to the generator L1 (hot) and of course N-N and G-G. Or just use a commonly available 30-50 adapter on the main cord.