Transmission slipping

My vehicle is actually a 1983 Winnebago but has the GM frame of a dually automatic Turbo 400. After sitting for about five years started driving it again after maybe 30-40 my transmission started slipping. It just kept getting worse and worse so it would not move it all forward. I drove it in reverse home for about four miles period after that I ended up removing the transmission and replaced it with another one out of another Winnebago that was two years newer than mine, same motors same layout and everything. Got it in filled it with transmission fluid and was off after driving about thirty miles of running shifting great. Same thing except for this time I did not even have reverse. Is it possible I did something wrong or maybe that transmission just was bad too? The steering wheel is not locked and the key will not turn it is locked. Came out of an RV that had the moving and not setting for any long amount of time. I think possibly I may have got a little bit of gasoline in the transmission fluid when I went to go check the transmission fluid so, I had a little bit of gasoline in a transmission fluid bottle. I might have got maybe a teaspoon of gasoline poured into the transmission. Would that have ruined my transmission?