Not trying to be disrespectful here, but I'm going to assume either you can't reach the fitting or your arm/body's to large to get into the cabinet enough for you to reach the fitting.
You can do a search within the plumbing section using the word "bathroom".
I came up with several listings. I want to say that someone within this forum has removed the sink to get some access to those fittings, but if your problem is you can't reach it then maybe someone with a longer arm might be able to help. I know it's not an easy fit and removing the cabinet door might get you some more space. I used an ice chest and laid on my side and that seemed to help me in the past.
What version of TM do you have that does help us forum members when answering some questions.
https://www.trailmanorowners.com/for...light=bathroom
https://www.trailmanorowners.com/for...light=bathroom
https://www.trailmanorowners.com/for...light=bathroom
https://www.trailmanorowners.com/for...light=bathroom