[Franco]

just my 2cents worth - *First go buy a cheap electric fuel pump and 10-15 feet of fuel hose. Cut a long section (7-10 feet) od hose and put it on the suction side of the pump and the rest on the discharge side - make a wiring harness with aligator clips on the end of 10 feet of wire. I would pull the fuel tank sending unit out, and the siphon tube from the fuel tank.
I'd look for a cheap C02 fire extinguisher - gas only, no powder and discharge it into the tank a little at a time - then I'd *put a mixture of simple green and water into the tank and swirl it around as best as possible - tilt the trailer so it all runs to one end and the insert the suction hose into the tank, you may want to substitute some metal tubing for some of the hose - and put the discharge hose in an old gas can- connect to the battery and suck all the old stuff out - it should only leave a small amount in the tank. Just my thoughts!!
Note - C02 makes it safer to work with, and keep the electrial stuff away from the fuel.