There may not be enough pressure to feed more than one faucet with hot water. If at a campground with low pressure, or medium pressure plus a pressure regulator, pressure can be low. Some recent models, I think, have an internal pressure regulator, so checking whether you have that and whether it is lowering pressure too much, may be one way to go.
The mixing valve may also be the problem. I found that the one in our 2008, after I had shut off the water to soap up and conserve hot water, would send out cold water for about 15-20 seconds (seems longer when it hits you) before I got hot water again. I learned to direct the shower away from me for that period. It could also be a bad cartridge on the mixing valve. Some mixing valves may have adjustments. And it could be a blockage in the hot water line to the shower. We have an external filter for city water because you never can be sure what campgrounds have and we have known people who got giardia. You need a 5 micron filter to remove giardia. It can also prevent blockages in the lines.
The Airstream is sold; a 2016 Nash 24M replaced it.