I recently went through a "though exercise" to determine the best way to feed a firepit from an Extend-a-Stay-type tee fitting. I concluded that using a 1-20 grill hose and a "last-chance steak-saver" adapter was the lowest-cost option that might work. I say "might" because I elected not to buy it and try it, and the fact that the firepit gulps a lot of gas. Still, it will probably work.
I believe I have considered about every option for making the Extend-a-Stay more flexible. It would be a long post if I send the pros/cons down. If you are interested in those details, send me a private message. I might consider documenting the options if it would help someone else.
One last note: I have an Outland 870 firepit. It's rated, like yours, at 58,000 BTU/hr at max output. That's more than a typical propane furnace. The firepit appears to burn about 1 gallon of propane in a typical use. I run it off a 20-pound grill cylinder, which means I get about 4 to 5 uses before the cylinder is empty. Tractor Supply fills it at about $2 a gallon. The propane truck at one RV park where I stay charges $5.20 a gallon. A 20-pound cylinder costs about $30 new. If I plan to use the firepit more than 10 times, it pays for me to buy a cylinder, rather than using the tank on the coach. I can also use the cylinder to run my grill, rather than running off the Extend-a-Stay tee. When I leave, I give the cylinder away, as I have no place to carry one safely on the coach.