I went through the pics of your other post and found this pic of the Tee with what looks like black stuff at the brass inlet fitting end.
I'm assuming this is the area you are referring to?
Assuming yes, by your description of the glossy black hard stuff at the fitting, the prior owner used a totally different thread sealant then they should have. They may have had a leaking issue and went with a heavy permanent sealant to stop a leak. They make sealants like that, mainly used in automotive/industrial use on steel fittings and not on home water plumbing. I'm not totally sure from here this is what you have, but by the pic and and your description, it is that type of sealant.
Assuming that is the case, odds are favorable you will break the plastic fitting trying to get it apart. And if you did manage to get it off without breaking the tee, there most likely will still be hard chips stuck inside the threads making it hard to seal up when you put the new inlet valve in.
If I was in that situation, I would give up trying to take the old tee off, or break it off if necessary to get it out of the hole in the camper. Buy a new tee and start over doing it right from the get go. The other gray pex fittings look like they used teflon tape on them, and they may come out and be reusable. But try to get them out without damage before heading to the store, as you might need some new parts to connect them back up if they are damaged in removing them.
Hope this helps,
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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