First of all, I see you list Texas as your home. Two years ago when we were there, they still scaled portable tanks, thanks to the railroad commission that sets the propane regulatons in the Lone Star. So they had to be removed from the RV and placed on the scale. Quite a few states now use the gallon meter and the scales are part of history. Very few times I have ever taken the fiver to the propane joint as many don't have much room. Propane here in Yuma, AZ. has been $2.29-$2.34 a gallon all winter. Of course the price will vary upon location.
For those that don't have your tanks weight. A few years ago in Monroe, MI they were filling tanks (the ones you swap out) at a large propane fill station/tank farm when one of the bottles had a bad valve and was over filled. The tank vented and a spark set the place on fire. 20 and 30 lb tanks go off like rockets if you've never seen a bunch of them in a fire. Those valve are not fool proof and anyone who know this will weight the tanks for there and your safety.
I had one filled once in cool weather. When it warmed up, I went to get the trailer and smelled strong propane. It was venting from being over filled. Took it to Suburban Propane and they opened the valve and let it drain for a while on a scale. They said whoever filled it had way over filled it. It does happen.
Also, would never fill a removable tank while it is on the rig. Saw one blow up once in 1995 when gas built up in the compartment while being filled. Something sparked it and an elderly woman in the RV died in the explosion. Our camper, parked 15 yards away had the windows broken and siding melted off as a result. Wasn't good and I'm scared of propane to this day because of it. Not something to mess with.
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The OP asked where to get propane filled. Out here on the left coast almost every UHaul dealer has propane. Also many gas stations, hard wware stores and a few markets. Call for pricing and watch the meter. I have had them start filling mine without resetting the meter to get an extra gallon or two out of me.
100 lb bottles don't have OPD so be sure if you use these the filler knows how to open the spitter vaulve.
I don't know anywhere that will fill removable tanks while they are still attached to the rig. I know the Camping World were we fill our MH has the scales and still weights the tanks. The service tech told me when he filled our 20 lb grill tank that you can over fill if you don't weight the tanks. Unlike the mounted tanks that besides the safety that have a gage to check.
Michigan State laws require portable tanks must be filled by weight, and I guess does not take into account that there are two other overfill protection devices on the tanks. This might be because tanks filled with propane from a 32F storage tank will have their propane expand a lot once it warms up, and it is more a northern states problem, that we don't have to deal with in the southern states where propane tanks sitting outside do not get that cold.
Most other states they do fill the tanks legally without needing to remove them from the RV, even the roofing companies have their "Portable" 100# tanks filled while sitting on the flatbed trucks. I have not seen a scale at a propane filling station in California, Oregon, Washington, or Arizona.
I have only seen a scale while in Michigan to refill a propane tank, and because the people would not fill the portable tank by the gallon when I was filling my motorhome(by the gallon), I declined to have it topped off, the place I was at charged for a full 20# of propane even though my tank was still 1/2 full.