After seeing the scary pictures by Rugged Brown, I decided to check out my bumper a little more careful. I have the same I-beam frame with the square sewer bumper, but Dexter (I presume) welded an extra ~2x4 channel iron right under the top of the I-beam. It is welded along the length of the I-beam, down the bumper, and it feels like it is just tack welded along the bottom edge....that would be good for drainage. I feel a bit of relief when I discovered that extra piece, and that may be a way of strengthening others members bumpers.
I can't get in there to take a very good picture, but here is the best I can come up with without crawling under the camper:
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263