Not practical to use an open end hose to dump. Generally hose fitting needs to enter pipe at dump. Not a coupler though.
It 'looks' like a 2 piece EZcoupler ell might fit within the door/tube opening. Its what we use but our box is larger.
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https://www.etrailer.com/RV-Sewer/EZ-Coupler/F02-3103.html
https://www.etrailer.com/RV-Sewer/EZ-Coupler/F02-3303.html
End of ell could be shaved a bit if needed and still lock into insert piece.
Went and measured ours. Camera prospective skews but hub against flat block, holding parallel, protrusion IS 3 3/4" at end of ell.
Shaving 1/8 to 1/4 off inside half of outlet opening to clear inside of tube seems might work. 'Course the insert piece wont and will need a place to live. We rarely use ours though unless at a site where the sewer outlet is a stub sticking up out of ground. Where sewer pipe flush to pad, most dump stations, ell with a weight on it sufficient.