"Thank you all" — wrong or right?

"(I) thank you all" is correct English, and a complete sentence.

"Thank you all" is correct because the "I" is implied. The verb "to thank" is correctly conjugated as "thank" for the subject "I." Correct as is!

The word "you", in English, can be singular or plural. This is different than in some other languages such as French where there is a distinction between "tu" and "vous" that can either be the formality of the situation or if the addressed are plural or singular.

So "you all" is correct just as much as using "you" to refer to a single person. As one commenter said it is compulsory in southern US dialects where they commonly run the two words together as in "y'all."

Thank you all sounds ok colloquially, but in correct written English a noun or pronoun must always be present, not merely implied.

I thank you all or I thank all of you would be my first picks.

For those of you not residing in the southern or southwestern US, "(I) thank y'all." is also correct English, and a complete sentence.