It's an extra propane hookup. I asked SMB to have an extra hookup installed like this, but what happened is they installed the extend-a-stay hookup instead. The extendastay is BEFORE the regulator and can be used to hook up another tank if your on board one goes dry or I can hook up another stove or grill. I pointed this out when I picked it up so they just tapped into the line going up to the stove.
There's a catch now. My little grill and stove that I wanted to use outside have their own regulators and when I hook them up to this extra hook up there's not enough propane volume for them to work properly (double regulated so to speak). They work fine when I hook them up to the extend a stay fitting. I had a grill that worked great when I hooked it up to the extra hookup, but it was made for RVs and it expected to be hooked up after the camper regulator.
But so far we've been using the green propane bottles when in camp and it's working out great. in fact it allows us to move it further from Sherman. I may just remove this extra connection because we really haven't found a good use for it yet. The extend a stay comes with a long enough hose to throw under Sherman and be able to set up a table to grill/cook by the side door if we really want to.