It's just a matter of plumbing connections. The tank side is high pressure, the regulator inlet accepts that high pressure. You can disconnect that hose (turn the tank off first!) and use brass plumbing parts to extend the inlet of the regulator up vertically above the regulator, and add a T connection there. For safety, I would add a ball valve to close off the extend-a-stay fitting. Choose a combination brass acme nut fitting that (should) have a spring loaded valve in the bottom of it, and then make up a hose that has two plastic acme nuts on it - one for the RV and one for the outboard bottle. Technically this is a double-ended hose, which is why you want that ball valve on the T - b/c when you connect the hose, you have just OPENED the valve in the acme fitting. There will be a pressure exchange, so having that ball valve closed (and the outboard tank off) will keep everything sealed until you turn on the outboard tank, and then open the ball valve.
There will be a pressure exchange with the onboard tank. IF you put the outboard tank upside down and above the level of the onboard tank, as the pressure exchanges and after it equalizes, the outboard tank will drain (as liquid) back into the onboard tank. This is OK - once the transfer is complete, the lines will flash back to full of vapor and the regulator is designed to handle either one, but it prefers vapor. If you leave the outboard tank on the ground and right side up, only vapor pressure will transfer and equalize.
When changing the tank, turn the ball valve off, turn off the outboard tank, and then it is safe to disconnect and change.
02 40' Monaco Diplomat: 1020 watts solar, Victron inverter. FASS, TRW steering, 23 cuft Frigidaire, D/W, W/D, Magneshade, Wood Floor, New cabinets, diesel heater