Auto soap & a soft scrub brush will work, just avoid the dish soap as it contains salt.
Have a look under the aircraft & see if there is a row of (probably blocked) 1/8in holes along the bottom to drain water... I once poked a bit of safety wire into a hole in the bottom of a P51 rudder & was somewhat alarmed to see at least 3 or 4 gallons of water pour out...
There are 2 kinds of Simple Green, one that is for aircraft & the other which is corrosive... Once you get it clean, keeping it that way will get easier... altho waxing a Shackleton might take a few days... I think you may need a Costco membership
If there aren't any drain holes in the bottom of the fuselage you could always add a few...Just don't use an electric drill..