I can't help with the primary question of why the gas portion won't light and stay lit. One suggestion, spiders love gas ports, check to see if one has built a Web in the port. Do not use anything solid to open it as I understand they are very sensitive. Use bottled air to blow out the port.
I am not sure why you switch to gas mode when disconnected from store power. The auto mode should take care of the switch for you when you disconnect from store power. We leave our Dometic in auto at all times and let the Internal system keep track of power source. If ac is available that is the default otherwise it switches to gas.