I would run it out of fuel or open the carb drain and drain all the fuel. I would then pour in a few ounces of sea foam and run it and drain the bowl again. You have now drained and flushed the whole system and no varnish or scale will form in the bowl. I would store them in a bedroom closet or something, NOT in the basement or garage. I wouldnt leave them at the storage yard.
I would charge the batteries and remove a cable so there is zero draw on them. I have stored batteries for several months with no problems. Sure, it's better to put a charge on them every month or two but as you say the batteries are $100 each not $1,000. If you cant lock them on the tongue box, move them into a storage bay or into the rv in a plastic box so there is zero chance of acid damage. Then tape a note on the door saying something like "Dont waste your time, there are no televisions or valuables inside". The next issue would be what your insurance deductible would be as compared to the amount of damage that would occur if someone used a pry bar to open the door. If there are no valuables in the bays, you may want to not lock them. Better to lose $30.00 in poo hose rather than a damaged door and trim.
If that generator never has old/stale fuel it will just about last you a lifetime.