Welcome to the forum!
Yup. Your experience is similar to mine. If you want to test fuses without removing them from the fuse box, get a fuse tester from an auto parts store.
They have a plastic top with an LED light inside. You simply push down onto the suspected fuse and make contact with the two contacts on the top of the fuse and the tester will light up if it's good. These testers are pretty cheap.
Now, let me tell you...you have entered the DARK side of RV repair. This is not easy stuff most of the time. RV's are designed by geniuses (mostly) and assembled by (sometimes) gorillas. You could use two additional tools. One is a voltmeter. You can use it to find a 12v source for power or tell if the wire to the appliance is getting 12v. The other is a long wire with a large alligator clip on one end and a smaller alligator clip on the other. You can clip the large one on the + side of the battery and touch the other end to anywhere you think should have +12v. This can help you figure out where the circuit is defective. NOTE: this is an unconventional trouble-shooting approach not to be used by people who are not knowledgeable about wiring, or who are accident-prone. I have used it when I couldn't figure out what else to do. On the ground side, you can also have an open circuit, so you need to look there, too. Do NOT use your +12v wire on the ground side of any load. You will fry something and maybe cause a fire.
2004 Tiffin Allegro 27.5 ft. P32 18,000 lb GVW. 8.1 liter. Workhorse chassis built May 2002. 24,500 miles. 2012 Jeep Liberty Toad. RVi2 brake unit.