Stopped in Reno for a few days where I picked up a new pressure protection valve to replace my leaking one. Also either called or visited 7 RV service facilities or truck repair services, and no one is willing to work on my diagnosing air system leaks in my U295 (or even just installing this new one) so it looks like I'm going to be doing it myself.
I have steel safety stands to place next to each of the 8 air bags, but my question is how do I completely empty the air system before I unscrew that valve from the dry tank on one side and the suspension/leveling system on the other? I can open the petcocks on the wet tank and the two dry tanks, but what to do on the other side?
And do I use teflon thread seal tape on the new valve threads, or something else maybe?
Use liquid Teflon not tape. Not sure what you mean by other side? If you drain both dry tanks and the wet tank there would be no other side unless you mean downstream and there would be no air there. The six packs would have pressure to the bags but that would have nothing to do with tank pressure.
"Other side" means other side of the valve, so yes, downstream. Why wouldn't there be pressure from the suspension and HWH leveling system on that side?
Raise your coach, put the safety stands in place, use the "DUMP" button on the HWH panel to release air pressure from the air bags. Coach will sit down on the safety stands. Open the drain valves on the tanks. All significant air pressure is now gone from system. Anything trapped between valves will just be a little poof of air - no worries.
For sealing threads on air system fittings, I recommend either Permatex or Loctite sealant paste:
https://www.oreillyauto.com/detail/c/permatex/chemicals---fluids/glues--adhesives---sealants/pipe-sealer---thread-sealing-tape/58b2d9326850/permatex-high-temperature-thread-sealant/per0/59235
Loctite 2087067 567 Thread Sealant 50 mL Tube, White: Amazon.com: Industrial... (https://www.amazon.com/567-Thread-Sealant-50-Tube/dp/B01N9ZH553/ref=pd_bxgy_1/141-4956838-8433715?pd_rd_w=G553D&pf_rd_p=fd3ebcd0-c1a2-44cf-aba2-bbf4810b3732&pf_rd_r=1FWQZQ40F27HDN16HM3F&pd_rd_r=d8355ecb-b8fc-4411-acaa-50938653009a&pd_rd_wg=rP3zE&pd_rd_i=B01N9ZH553&psc=1)