Yes, they refill propane tanks. Anywhere from the 20lb tanks to the 100lb tanks, the brand of tank doesn't matter. They cannot, however, fill RV tanks that are permanently apart of the RV; they're not certified to nor do they have the equipment.
I'm not sure if they actually refill. I do know, we drop off our empty tank, and get a full one for our grill. They are about the cheapest place around for propane. I hope this helps.
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