During cruise flight at 7,500 ft, the RPM started to decrease until the pilot stated that 'it was just like you ran out of gas.' The aircraft collided with a wooden powerline pole during the night forced landing and came to rest inverted. Investigation revealed about 20 gallons of red avgas in the right tank and 1 gallon in the left tank. Approximately 3 ounces of liquid were drained from the carburetor, which analysis concluded the majority of it to be water. Maintenance records revealed that wing tank sump drains had been installed in May of 1983 and the AD dealing with water contamination of the fuel system caused by wrinkles/folds in the tank bladders had been signed off in September 1984. The mechanic that performed the work was contacted and stated he found no wrinkles that would justify installation of the Cessna drain kit described in service letter SE84-9. Examination of the bladder tanks revealed wrinkles/folds in the vicinity of both outlets feeding fuel to the engine.