I have trouble with a propane nut - someone screwed it on insanely tight. I also actually sometimes have a hard time with the cap of my dump valve because it can be stubborn and its diameter is so big that it calls upon the last joint of my fingers to have the strength needed. (In fact, sometimes opening a new jar of pickles gives me trouble though too.) But specifically there are a lot of knob or wheel turning parts on an RV where hand strength comes into play.
For the dump valves I bought Camco's Dump Valve Wrench and honestly it's not very good. It sure doesn't work on the cap. So yesterday I was sniffing around on Amazon and happened across a thing called a Strap Wrench. I'd never heard of it. I checked it out on youtube and there's a "how to use" video that's very short. Seeing that? I went back to Amazon and looked for the strap wrench set that has the best star ratings on the reviews, and it's Craftsman. Two size wrenches (each adjustable to fit) - a small which some reviews say will even open a prescription bottle size cap -- and the large wrench goes all the way up to over 6 inch diameter wheel/knobs.
Here's the how-to video which gives you the general idea what it is and how it works...
https://www.youtube.com/watch?v=eoRT8UvYraI&t=6s
...and here's the Craftsman set of two. For $12 for the set of two... sheesh.
https://www.amazon.com/Craftsman-Rubber ... wrench+set
Mine should arrive on Thurs. so as soon as the rains are done I'm going to try it on my freshwater hose connection AND on the dump valve cap AND on that nut/knob and will come back with whatever results it gives.
For all I know you all know about these and I'm the only one who doesn't yet, but if not? We all should.