Extend-a-stay, extend-a-flow or Sturgi-Stay type fittings are easy to install if there is space to do so. They are placed in between the tank and the regulator.
I added one on a Class C and it was very useful to have in Winter. I ran the furnace off a portable BBQ type tank and it was a lot easier to exchange or fill the portable tank than moving the RV which we were living in while doing house renos.
They're also useful to run camp stoves etc off the main RV tank.
Another option if there's no space available to add a fitting could be by using the main tank fill inlet. I have never tried that method but have read of others doing so on the internet. Apparently it work as the gas pressure equalizes between the two tanks. It will not fill the main tank with liquid propane. Like I said - never tried it and you'd probably have to get a specialized fitting made.