http://www.flickr.com/photos/31753213@N04/sets/72157634784197322/Pix of jobThe job is done - on closer inspection I noticed that there is a plastic belly(?) pan across the back of the camper, similar to what AnEv942 describes (maybe it should be called a butt pan, considering what part of the camper's anatomy it covers). Removing the pan required unscrewing a dozen or so self tappers and a few square drive woodscrews - guess they used whichever was handy. Once that was out of the way it only took a few minutes to change the valves, not counting the time it took to figure it out - and the time it took to do a leak test - put some water in each tank and left a piece of cardboard under the pipe - no drops on it after an hour - good enough for me.
I had a bayonet mount slide valve on the camper for a few weeks before I got around to doing this job - plan to carry it along from now on in case this happens again - yet another emergency item to carry but it is sure nice to have when needed. It provides another line of defense against nasty surprises when removing the sewer cap.
Here are a few pix - the first one shows the original valves in place
http://www.flickr.com/photos/31753213@N04/9361414952/" title="IMGP1313 by Jayelldoubleu, on Flickr">
Tried to post a few more pix but flickr has changed the rules apparently - finding it too much trouble right now
OK - added a link to the pix on flickr - done with this post now -