As others have said the Honda EU series (I'd go with at least a 3,000) can suck gas out of an external tank.. SHOULD be able to adapt it to suck gasoline out of the RV's tank via the geneator port just like the Onan but if you do be sure to put in a shut off valve.
ONe of the more unique things about the Honda Generators (At least the EU series) is that they have a genuine fuel pump. and thus can "Suck" gas.
I would use a motor boat fuel hose (With primer pump) so you can loosen the cap and fill the on-board tank by hand pumping.