They are easy to refill and very hard to over fill (unless you put the bottle in a freezer which will allow it to hold too much propane).
I refill mine all the time — I have an empty cylinder which I put on a scale and zero out the weight, then I weigh the partially full cylinder, add propane (I purchased a filler on Amazon that attaches to a 5 gallon propane cylinder) and add propane. It takes a few minutes and I check the tank while the process is underway...
Why do I do this?
Mrs. BBB333 & I have a RV that we use but also, on occasion we rent it out (via Outdoorsy) and I charge $5.00 per propane bottle to refill. We live north of YNP and Teeton National parks and these little cylinders cost $12.00 in the tourist areas. The renters save money and the bottles work just fine.
Screw California!
I would never rent out my RV. Just what I need is some bozo wrecking it. Plus the wear and tear. Every bump is entropy. The amount of work I would spend fixing it would exceed the amount of money I could make renting it. This is the meaning of “cast not thy pearls before swine”. The RV is the pearls and the renters are the swine.