An "extend-a-stay" is simply a BRASS TEE with generally suitable BRASS FITTINGS to mate up to common LP connections. Make your own.
First, get the hose you want to use to hookup your extra LP tank/gas grill. You need to know what fitting is on the end.
Turn the LP off. Take the regulator off of the camper from the tank valve to the LP tubing.
I would post a picture that shows how I set my homemade auxiliary LP connection up. But I don't have a image hosting site. So hopefully my little diagram will turn out right. I tend to use this set up to run a tabletop gas grill (without having to remove the grill regulator) since I have set up my LP system on a manifold (and run several 20LB LP tanks OR a 250 gallon tank). The LP quick connect is from an LP hose that came with the fittings. You can also buy the 2 piece fitting at places that sell LP fittings, like propane stores, ACE hardware, Tractor Supply, Amazon, etc. Do not confuse gas quick connect fittings with air quick connect fittings. The air ones leak.
Existing Tank Valve <---> BRASS TEE <---> Existing Regulator
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GAS SHUTOFF VALVE
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GAS QUICK CONNECT
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Hose to extra tank or gas grill
FYI: when I run off a 250 gallon tank, the gas company always checks to make sure I have a larger regulator on my bus. I do because I installed a residential LP fireplace and once I added up the burner outputs on my residential gas dryer, my residential gas range and the residential gas fireplace, it surpassed the capacity of the tiny regulators that are used on RVs.
So if you do this with the intention of renting a big tank, you will want to place the "T" AFTER the tiny regulator on your RV. OR simply add a second "T" with a shutoff so you can use it later without having to install. The gas company will install the big regulator ($) on the big tank if your onboard regulator isn't big enough.
I fill the tank once every 8 months but we aren't in that cold of an area. I am in "planting zone" 7a (lowest temps are 0 to 5 °F) for reference. the last few times I've paid rent on the tank, it was under $15/yr. We do have to remind them about the tank rent. They don't send us a bill. We also pay for the gas upfront when we tell them to come out to fill the tank. We wait until it's dead on 20% and have it refilled to 80% (the max). That gives them time to come out and fill the tank without being pushed to have it filled the same day due to a winter storm. Never run a big tank to empty. You will be charged to purge the air from the lines AND for a line inspection.