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Several people store them inside plastic totes, and then haul them either in the trailer or the tow vehicle.
As mentioned above, some convert the generators to run on propane fuel, to eliminate having to deal with gasoline fuel at all.
Some install receiver hitches on the front of the tow vehicle and carry the generators on a hitch-mounted carrier.
There are a few vendors that offer mounting plates for at least one gen set on the tongue of the trailer, behind the propane tanks.
Jeff & Cindy
'09 27FB Flying Cloud
'91 350 LE MH