Filling the small RV tanks is always higher per gal than home tanks.
My 250 gal home tank is now $1.64/gal, 5 gal portable tank is $3/gal - same company.
If you have a home tank, SOME trucks will carry the adapter to fill the small BBQ tanks. Back when we had a house, we had a 500 gallon tank that we owned outright (meant I could get LP from whomever I chose). When I would have them come out and fill, I would ask if they would fill my empty BBQ tanks. If the driver said yes, I could get them filled at the same price per gallon as my big tank. Once my big tank was full, they would stop the pump, change nozzles and fill all my BBQ tanks too. I had about 10 or so BBQ tanks at the time. I would ask when they came out to pump and then line up all the tanks next to the truck. I thought it was nice of them.
In several of the towns we have lived in, the LP companies would have a customer appreciation day (usually Saturday) where they would have balloons and drawings for grills etc. It was generally for their anniversary of being in business. The last time we filled up BBQ tanks (filled up 5 tanks all total) it was $6 per tank. That was about half price. The LP company that we did this at has this once a year, every year. My daughter rounds up all the LP tanks she can find.
I have heard (but not had the opportunity to actually do this myself) that if you haul in 5 BBQ tanks (20 lb) some LP companies will fill those tanks as one and charge you the 100 lb tank rate. Like I said, I've not had the opportunity to try this. I have noticed that more LP companies are charging the same price per gallon for 100 lb tanks as they do for 20 lb tanks.
We are pretty frugal with our LP use since we don't have any RV appliances that are using LP. My LP range is a residential unit and is very economical on LP. We go thru less than 100 lbs LP per year (fulltime).