William,
I will see if I can help on some of your questions.
1. I am not sure how AquaHot wires this fuse, so you will probably need to get Orman to help in diagnosing this problem, but it sounds like you may have a wire shorted somewhere, unless the fuse is for the combustion fan inside the AquaHot.
2. Your coach holds 100 gallons of fresh water and it has an automatic shutoff that closes the electric fill valve when the tank is full so that it will never overflow unless there is a malfunction in the system. The first thing that you need to do in diagnosing this problem is to recalibrate the sensor. If recalibration of the sensor does not repair your problem, you will need to replace the sensor.
3. I am not sure where the outside temperature sensor is located. It is probably behind the front cap on the left side, but we will need to wait for a 2003 Marquis owner for input here.
4. The shutoff valve for your Insta Hot should be located in the compartment with your water pump (right rear of your basement).
Gerald