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This is probably because the generator is starting off of the house battery. You will need to wait until the house battery has a charge to start the generator.
Now that you say this it sounds like this might be a problem with the coach not charging the house batteries. Have you checked the house batteries with a load tester? It is possible that this could be a problem with the batteries themselves not being able to take a charge.
Interesting, it sounds like there is some sort of battery isolator that is not working correctly that will not allow the coach to charge the house batteries.