If no leaks, check to be sure each valve is in correct open position so that water can flow through. Valves in RV's can be confusing as to what is open and what is closed. It took me a while to figure out the correct position of each of the valves in our first pup but when I did I used a permanent marker to indicate Closed "C" and Open "O".
  Are you using the 12v water pump or hooked to city water connection? If a water pump do you hear it cycling on when a faucet is open? Is the switch on for the water pump? If the switch is on and the pump is not cycling you may have blown a fuse.
If water is running behind the shower fixture there may be a broken line so the system is not pressurizing to allow enough pressure at the faucets. If your pump runs constantly and doesn't pressurize the system and then cycle off when pressurized then you have a leak somewhere.

Of course, I have no clue about your particular camper, but my first thought was ..... is the city water backflowing into your holding tank somehow?  If so, it would make sense that you have to fill the holding tank (an hour?) before you get water form the spigots or faucets.  Just a thought to check.  Look for a valve that might bypass the pump and allow a fill from city water.

So you have two issues?

#1 Almost no water pressure when trying to use the city water connection?

#2 An apparent leak somewhere behind the shower valve?

Since you say all is fine when using the fresh water tank and 12V pump, the problem sounds like it is with the city water backflow valve, or the piping between that valve and the remainder of the plumbing system. Typically they connect that city water connection into the nearest cold water line. Look for a T on the inside of the camper. I have seen it inside a kitchen cab or bath vanity, under the shower base, in the compartment next to the water heater etc. It will be close to where the connection is on the outside of the camper, as they do not like to run extra lengths of pex tubing. If you are sure that the city water you are using has pressure, It needs to have at least 20 to 25 lbs pressure to open the backflow valve, the backflow valve in the city connection could be stuck closed. Pry out the screen and behind it in the center should be a small white pin that you should be able to push in with a screwdriver, its spring loaded. That is the valve that prevents water from coming out of the city connection when your using the 12V pump. The purpose of the pin is so you can open that valve while winterizing with the pump, and let the antifreeze flow back through that connection to remove all the water. I have seen them stuck in both the open and closed position.

If that backflow valve is not stuck closed, then your problem most likely is debris blocking the line somewhere between that valve and the rest of the plumbing supply system. The only way to check that out is to start taking it apart.

Not saying some manufacturer has such a system to fill your tank from the city connection but I have never seen one.If you were leaking back into the tank while on city water eventually the tank would fill and run out of the gravity fill connection. When your on city water your 12V pump should not be needed. If water leaks back into the tank then you have an issue with the backflow valve which is usually part of the 12V pump assembly.

At this point I would be more concerned with the water coming from behind the shower, that sounds like a loose connection but even so should not cause the pressure problem you describe. For a leak to cause that it would need to leak a lot, as in like flooding the camper.

[quote author=bikendan link=topic=2907.msg27960#msg27960 date=1394043957]
because it's a new RV, debris from the factory, gets into the water lines and clogs faucets and showers. this is very common. sawdust and foam flakes are often the culprit.
try taking the faucet screen out and check for debris. same for shower head wand.

I have also seen plastic chips from installing fittings in fresh water tanks, in the tank. Those usually get sucked up into the pump filters. Its a shame but the manufacturers pawn the final cleaning off on the dealer. Some dealers do a great job, some not so much. Typically none go into non accessible (easily accessible) areas to clean, and that is where most of the construction debris ends up. If you have an enclosed underbelly pull that loose and see what comes out, but that's for another post.

The way I take Alabamas 2nd post, where he says "Water pump works fine. Will have to use holding tank and pump until we can get to the shop I guess" he has no problems when using the 12V pump and water tank. That would rule out clogs at individual fixtures.

Usually when its loose debris the first thing that clogs is the flush valve on the toilets. That is where the smallest opening is in the entire system that water has to get through. When that clogs usually you can not get the toilet to shut off, the debris holds the flush valve open.

Alabamacamper, did Camping World run everything for you both from the tank and city connection during the PDI, or did they rush you through it. Camping World tends to do the rush method.

that's why it's important to demand a thorough PDI and make sure that your PDI list is covered, before you leave with the RV.
many dealers will rush customers through, if they allow themselves to be treated that way.

there are tons of RV forums out there to help customers know what to do and what to ask and what to expect.