I have a 1993. Not sure about this one, but can't imagine they got a lot more creative.
My fresh water tank has an inlet at the back then labeled fresh water. It has the vent overflow mentioned here. Frankly it almost always leaks
a bit in travel. But otherwise generally only has water flowing through it when you do over fill. In your third picture the blue hose is probably the vent over flow. Then I have a drain valve and line. IN the fhird picture that is obviously the valve there. Identical to mine.
I wonder if your line at the top, the black one in the third picture is the fill line. You should be able to trace it and figure that out.
I think you are missing a line or a picture of a line. And we seem to be missing the pump. The pump should as you said have no seals or anything. And from what I can tell a 1`6 foot scamp
should always have an electrical
pump. Now the obvious question does the pump make noise. They do make some noise, more if they are running empty. And they do freeze up especially with no use. If it is not making noise that is your problem. either it is not getting power or it is frozen up. Probably the second.
Your pump should look like this https://www.scamptrailers.com/parts-...mp-detail.html
Now you are talking also about plumbing under the camper. The pump should be in the camper body. Frankly mine is right in front of where picture 3 seems to be showing. The pump should have its back end pretty much right at the left edge of that picture and then run on to the left. It should have an input from the tank and then run into the camper plumbing. It sounds like you don't have that or have not located that.
Now you are talking about a pump under the camper. There is one. It is the shower gray water pump. It takes the water from the shower and pumps it into the gray water tank. It actually should look a lot like the fresh water pump.
I think you need to do some more research.