I've been pouring vegetable oil through my drains to lube the sewer valves. This wastes a lot of oil as you have to fill then overflow the P trap before the oil actually gets to the gray tank. My first thought was to drill a hole in the sewer line between the gray/black and master valve then use a funnel to pour the oil in. (see 2nd photo) I haven't done this since I haven't come up with a foolproof way of sealing the hole to keep the black water in during dumping. (a must for me!)
While dumping today it hit me! All I had to do was rotate the 90° elbow 180° and voila! I just close the gray & black valve pour in enough oil that it will reach the gray valve (the gray valve uses a 1½" pipe into the 3" line see 2nd photo) then exercise the gray valve then the black then the master. Once the valves are sliding smoothly I close the master valve put a pan under the elbow to catch runoff and twist it off. Very easy and quick!
This will only work if your dump systems is set up similar to mine but hopefully this will help with a few of you out there! Don't overlook the obvious!
see ya
ken