I installed a "Stay a While" tee that is intended for that purpose. It is made by Mr Heater and is available in many RV stores and on Amazon. It also has a hose that can be connected to a propane grill with a built in regulator. See
HEREAnother brand - Extend a Stay by Marshall Gas Controls - was popular but they quit making it for a while. It appears to be on the market again at Amazon.
A very similar one is made by Sturgi
HEREOne advantage of these over a home made one is some built in safety features such as check valves.
They install between the RV tank and regulator.