I'd first start the main engine (assuming the chassis battery isn't dead too--if it is, jump start it normally). The battery combiner will supply 12V power to the house 12V system. If the levelers will not raise then, try disconnecting the negative from the house battery in case it's "sucking down" a lot of current that you want to use.
If the house battery isn't charging from being plugged into shore power, it won't charge better from the generator, since in both cases it's the same converter doing (or not doing) the charging.
If you look in the literature for the levelers, there might be some sort of a manual override/retraction method you can use with a socket or something. I don't know how common that is, nor for that matter what particular levelers you have, but it sure seems like a sensible feature to have.
