That should be easy to locate. Outside, down low. Usually black, 3" diameter. Has a cap over it (or should). The unit will have 2 pipes running into it. One usually 1 1/2" or 2" for the grey water and a bigger 3" for black water. Made out of PVC plumbing pipe, just like in a house.
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If you pull tabs have not broken. (it does happen) They are locate on the right side on the bottom usually around the outlet pipe.
That should be easy to locate. Outside, down low. Usually black, 3" diameter. Has a cap over it (or should). The unit will have 2 pipes running into it. One usually 1 1/2" or 2" for the grey water and a bigger 3" for black water. Made out of PVC plumbing pipe, just like in a house.