The Navion gives you a choice between a natural gas or a diesel generator.
Get the diesel genset. This way, you have two fuels available for heating with, both propane and diesel used with an electric heater.
I live in Texas, so things are different, where a generator is a must for a good chunk of the year.
I do recommend a solar charging system, but always have a generator as backup. One never knows if they woke up with flat batteries, and have to be scooting out of an area late at night.
As for portable generators versus an inbuilt system... the inbuilt system is nice because it is a lot less visible to thieves (I've yet to hear of a thief dropping and making off with an Onan, but I hear about portable generators disappearing all the time.) You can also get a Magnum Energy genset controller to have it automatically start up when the house batteries get to a low voltage. You can also set quiet hours as well, so the generator won't start up and break a CG's rules.