Is there enough magnetic power for a sheet magnet such as 5'x1' truck sign to erase tapes if it is draped over a camera bag with tapes inside?

Possible, yes - Likely... not so sure. Assuming the tapes are in their cassettes, and the bag is between the sign and the magnets, the risk would be minimized, but not negated

You can always test the premise with a scratch tape and magnet.

I hope (and I'm assuming) that you are asking because this happened to you already, not because you do this as a standard practice. Aside from the possibility of erasing the tape, introducing a magnetic field to any of your video and audio gear could potentially cause a problem. Why risk it?

Oh no. It hasn't happened to me at least by me doing it but almost did on a project I was working on. The magnet signs were placed in a seat near some tapes in a case by a p.a. and I saw it and rushed over and moved them farther away. Noone else thought they were strong enough to do any damage. To me, any magnets near tape is not good, not even cell phones and pagers which both have magnets to make them vibrate.

While you should never risk putting magnets near your equipment, those car magnet signs are deceptively weak. Their magnetic ability is due to their large area of a "weak" magnetic surface. I doubt that they would have done damage, but I too wouldn't have risked it to find out.