dznf0g... always good to hear from you.
I struggled for a while with the same question though in my case it's to run a propane grill. I tried a couple of things.
The first thing I did is get an 8' hose with an ACME connection on one end and a fitting on the other end to match the grill. My grill uses those disposable 1-pound cylinders so it has that kind of thread. But it doesn't really matter, there are adapters to go back and forth:
Camco RV Portable Campfire
The idea was that I'd disconnect one of the 40# cylinders, run the furnace and whatever else on the other one, and just hook up the hose to the 40# cylinder valve.
It didn't really work out. The hose wasn't long enough. It was a pain to hook up because there isn't much clearance in there, and you have to remember to switch the regulator to the other tank because otherwise it will leak gas when you disconnect the pigtail. Worst of all, I couldn't let either tank run out completely. That's hard to do unless you just don't use much propane.
So I got one of the extend-a-flow kits. You have to look around to find one with ACME threads, and it's important to get the kind with a 90 degree turn built into it since they fit better. Tweety's has them:
Marshall Brass Acme Extend-A-Flow Tee Adapter
It fits on my right tank without any modifications. I generally set the regulator to use the left tank first. If the right tank is less than 3/4 full or so when I refuel, I either have the dealer top it off too or move it to the left side so that I always start with a full tank on the right. As a result I only need one tee.
To put a tee on the left side would require installation of a longer pigtail hose on the regulator because the tee has to point a certain way for the auxiliary outlet to point up.
The tee comes with a brass cap with a gasket, which I install whenever I remove the hose. That way there's no concern about any leaks that the spring-loaded valve might develop (they sometimes do get them).
I got a 12' hose this time which works for me. When I'm using the hose I route it down inside the tank enclosure. There's enough clearance to attach it to the tee without excessive bending. The 12' hoses are readily available, and you can connect two or more of them end-to-end if you need more length.
Overall this arrangement has worked out for me. There are two inherent problems with high-pressure propane hoses though that you have to be aware of:
1) The longer hoses will hold a considerable amount of propane, enough to pose a safety hazard during storage. So you have to bleed them by shutting off the tank valve and running the appliance for a while, until the gas is used up.
2) The hoses can fill with liquid propane if you leave them connected overnight while not in use. What happens is that the hose cools faster than the tank, and so the tank pressure is high enough to make propane condense in the hose. That can make the regulator on your grill or campfire in a can or whatever to frost up and behave badly. The fix is to disconnect the hose at the tank end when you're done with it for the evening.
I've used the low-pressure quick connect systems in stick houses over the years and they work great but take some effort to install and get right. The main problem is that you have to be sure that the quick connect has enough physical support to withstand the hose pulling on it since eventually people or dogs will trip over it and these connectors don't have a breakaway feature (they should). As noted upthread you have to remove the regulator on the appliance you use, which can be problematic for appliances that use a nonstandard regulator. If you have something that uses a lot of gas you may exceed what the piping on the trailer can deliver without reducing the gas pressure to the furnace, range, and fridge.
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