P203F - This is the sensor that tells the computer how much DEF (diesel exhaust fluid) is left in the tank. It means that it thinks you're low on DEF. This either means you are, or the sensor in the DEF tank may be bad.
U010C - This is typically turbo actuator failure. Not super common, but it does happen. City Diesel sells a replacement actuator kit. Does the truck build any boost, and does the exhaust brake work? It could also mean a bad turbo, but it's more often the actuator than the turbo. If you don't run the exhaust brake to exercise the actuator arm it can stick due to soot build up. You can google how to test for which is the issue.
P0340 - Doesn't necessarily mean a cam position failure. It is a cam position sensor "A" CIRCUIT malfunction. If you replaced the cam sensor with no effect, the likely problem is the wiring.
Heater cores are an issue on this truck. Pretty easy to attempt a flush. If it doesn't work then you're out an hour of your time and you have to change the core. If it does, then you're golden.
Did you just buy this truck? Seems like someone was letting things sit without fixing them and allowed issues to stack up since none of these are related to the other unless on the super slim chance that you have a bad pcm.