This has really opened my eyes....I ,after 55 yrs turning wrenches never dreamed of this....I have searched and researched the internet and YES bleach will destroy your gas or diesel equipment if it is pumped thru it......including possible explosions, and engine seizure.......DRAIN it..no matter the cost and no quick fixes like water or such.....
You can bet I will be ordering me a locking gas cap from NAPA Monday.....to guard against someone doing this to me on a trip.
Iam sorry for your dilemma however its not the end of the world if you drain , and remove the tank for cleaning AND DONT TURN YOUR KEY ON to activate your pump. I would think a pump is in the tank..you will also have to replace it.
When I owned a union 76 service center a farmer once put gas in his radiator ,so you are not the only one.