Op... I was typing same time you were posting. Upon seeing your pic's check out links at
http://www.valterra.com/product-category/rv-products/aftermarket/sewer-sanitation/What yo need might be under different name, also check out "FLANGED VALVE FITTINGS" see example pic's at
https://www.google.com/search?q=rv+flanged+fittings&biw=1301&bih=589&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiFpZW9kcDSAhUI82MKHd66CMEQ_AUIBygCIf worse come to worse you might have to put in a bulkhead fitting. To do that you'll have to drop tank and install a access porthole if you can't reach with your arm. Basically drill a big hole so you can get arm and wrench inside tank to tighten bulkhead fitting and then install cover for access hole you drilled.
https://www.amazon.com/JR-Products-31025-White-Access/dp/B004RCSKZE