Jim
Looks good and cam locks do provide a secure seal but how do you plan to handle any tank valve leakage? I have a tiny bit and added the additional valve on the outlet so I don't have any surprises when I uncap.
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Lyle - I hear you. I've had the CamLock system on 6 coaches now. And there have been times where I've had some "sewer cap surprises". Generally, any liquid I've had behind my cap has been residual that was left in the lines, past the tank gate valves - likely due to insufficient fall on the lines based on install or pitch of my coach.
As a matter or course, I generally have the sewer hose camlock end in my hand while I remove the sewer cap with my other hand. I hold the sewer hose below the sewer output to catch anything present there. Usually very little to none.
This all said, with the coach I just turned in, I was getting some "black tank surprise" a while back. I ended up having the black tank valve replaced. After they removed it, it was obvious there was toilet paper impacted into the blade slot. We changed the valve anyway as I had one with me.
Then more recently, my galley gray tank was not sealing well. I turned the coach in like that and am having the plant replace that valve. But with this valve leak, it was the most problematic for me. We were on the road much of this past January and February and found ourselves in sub-freezing weather many times. On freezing days, I could not leave my sewer hose hooked up as the water from the galley gray would constantly drain as the sink was used. That could have caused a frozen mess. So we had to be careful.
To my knowledge, neither Valterra nor Camco make a plastic 3" external style gate valve with a male to female camlock fitting. I think one could be Frankensteined together with adapters but it would really extend the discharge outlet. Perhaps one day Lippert will make such a valve.
On edit: Let the Frankensteining begin 
3" Male Camlock x 3" MPT -> 3" Gate Valve 3" FPT x 3" FPT -> 3" MPT x 3" Female Camlock
So - there are 3 off-the-shelf parts that could do the task of providing an external gate valve with Camlock fitting on both sides (M to F). It would probably stick out at least 6" more than a bayonet x bayonet valve.. Perhaps the sewer outlet could be cut back a bit to minimize the projection...
Further edit - Frankenstein Light
Perhaps there's a better way. Purchase a Gate Valve 3" Slip x 3" Slip. Hack it into the 3" line that is parallel to the frame before the sewer line makes the 90. Or to make the valve more "replaceable", purchase a Gate Valve 3" FPT x 3" FPT and use 3" MPT adapters. This way, you can spin the valve off (I guess
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Materials Sources:
3" Camlocks -
link
3" Gate Valves w/ Plastic Paddles -
Link to Mfr site ---
Link to product on Amazon.com
