Thank you for the reply. Yes, I've run into similar challenges on getting a more user friendly access panel.
Plan A is to cut out a section above the dump pipe the same width as the dump pipe so the entire panel can slide down and off once it's unscrewed. I can fab a cover for the missing section pretty easily with some leftover plastic I have lying around. If that doesn't work I may just permanently mount the factory cover and just cut out a smaller square access hole and fab a new cover.
Either way I'm happy to say that inside of the cover the valves, insulation and piping are in great condition considering the age of the trailer. I bought this from the original owner, and he didn't remember ever having the valves changed out so there's a chance they were original
Thanks again for the reply and info!
-V