You are not stupid for trying to change out the faucet. I commend you for trying it. It is very simple. How far did you get.
Did you still smell gas after you shut the tank off?
As Lou said, propane is fed through either black iron pipe or copper tubing. Looking at the top picture, the copper line over on the right side is probably the gas line for the water heater.
Where you're talking about kitchen sink, you may have a air admittance valve that may be leaking a little and the smell of gas may actually be coming from your gray water tank. That valve would be located on you sink drain line. Follow the drain line away from the sink. It will probably go to a "Y". The piping will continue down and it will also go up maybe 1'. The valve will be on the end of that top pipe. This is just a thought if you still smell gas after shutting of the propane tanks. I know it's a new 29 foot class c Winnebago Aspect 27k so you could have some rotting food in the gray water tank left over from last season.
That black box looks to me like it's the electrical box protecting the backside of the water heater 110 Volt heating element. If you ever burn out the element like when some people turn the power on without having water in the heater, that's where you'll have to go to change the element. Note: They can be purchased at Home Depot
