Will I need to show proof of funds or onward travel when re-entering Thailand with a Non B visa and work permit?

If you have RE ENTRY PERMIT they don't ask you to show proof of funds. Before leaving immig. passport control, check your passport make sure the officer put the right date that match to your visa expiration date.

You WON'T be asked for proof of funds. I have never seen people on yearly extensions asked for proof of funds when re-entering the country.

I would err on the side of caution and at least take a copy of your work permit. There is no rule that I could find that states you need to have it on you when you return to the country BUT like Robert I too read reports that mentioned people being asked for it.

never asked for my work permit in 12 years of using re-entry visas.. or applying for re-entry stamp..

Robert Lagas Disagree with what you are saying. If on multiple entry non B maybe, since multiple entry visas are designed to travel into Thailand many times, but extensions of stay are for living in Thailand for that period of time.

Proof of funds would be idiotic. "I have a work permit, I get paid a salary, what proof of funds do you need?"

Exit ticket: "I'm not planning to leave , I don't have an exit ticket, I am planning to extend my stay for another year, so I don't want to go anywhere".

Proof of WP may be requested though, rarely.

I've seen comments here that people returning had to show the WP, others did not get asked. Immigration is allowed to ask for the three proofs to everybody who enter, but most people do not get asked for anything. It really depends on the Immigration Officer, it is unlikely you get asked just based on more people enter without questions than with questions. But if asked you will face a problem if you can not provide.

No you don't need

I have gone out many times with reentry permit and have never been asked to show anything.