[JRTJH]

I would first carefully inspect the white hose from the fresh water tank to the pump inlet. A cracked fitting on the pump or a bad O-ring on the fitting can (and does) cause this kind of issue. A cut in the white hose or a loose worm gear clamp can also cause the issue as can an "abrasion or wear" where the hose goes through the floor and towing can cause the hose to wear through.

That said, I have seen an occasional water pump with a "pinhole in the diaphragm" that as the pump operates, that pinhole causes the pump to stop/reduce the water volume delivered to the plumbing.

I think, in the past 30 years, since the diaphragm pumps replaced the piston pumps as the "norm", I've seen 3 or 4.

Essentially, what I think happens is the pinhole allows the pump to "lose suction" and the amount of water "pulled into the pump decreases" so necessarily, the amount of water "pushed out of the pump also decreases"...

I'd start by filling the fresh water tank as full as possible, then turn on the pump and open a faucet that has been involved in the decreased volume/pressure. Let the pump run as long as there is water in the tank and see if there is any "decrease at a specific tank level". If you don't see a decrease, refill the tank and try with a different faucet that has been a problem in the past. (remember that you'll be filling your gray tank as you empty the fresh tank, so plan accordingly).

If there is no "revealing malfunction in the above tests" then I'd suspect the pump diaphragm is a good contender for the answer to "why"....

Your trailer should have a FloJet pump. They are a "good brand" with a 2 year warranty. You can buy a replacement on Amazon for around $45-50, so buying a replacement (if you're out of warranty) is not a "outrageous price".