Pressure Tank Install

I am currently trying to install a pump and small pressure tank to use with an RV. I thought I knew what was going on, but it doesn't seem to be working.

I have everything connected but it seems like the pressure tank isn't doing anything at all. It doesn't feel like there is any water in the tank and as soon as you open the faucet the pump comes on.

What could be happening? Did I mess up the pre-charge, would that cause this issue? I thought I got this right but probably didn't. My pressure cutoff switch is part of the pump. The first issue I had is that the pump is not new so I have no idea what the pressure is set at. I tried to find out by capping off the plumbing before connecting the pressure tank, but I'm not sure I did this right. Is there any way for me to figure out the pressure? I'm including a picture of the plumbing for reference.

The pressure needs to be set the tanks come pre charged at 40psi. Also you are going to have issues between galv pipe and brass. The two metals will eat each other causing electrolysis

I did some reading yesterday and it said I needed to set the pressure to 2-4 psi bellow the pumps cut-in pressure. Is this correct?

Also thanks for the heads up on the fittings. I should have looked into that before hand. What are my options here? I will pull the galvanized off and replace it. Is it just galvanized and brass that the issue?

That looks like a thermo expansion tank not a pressure tank.

Where is the pump? Your correct on the tank pressure, two pounds less than the pumps turn on pressure. If this is a jet pump, you won't be needing that pressure relief valve.