I recently took my two 30# tanks to AmerGas in Knoxville for recertification after 12 years.... they cahrged me $10.... one of the tanks was full.... no ptoblem... the 12 year recertification is a visual inspection.... test the valve for leaks, etc.
Rust alone makes the tanks unsafe... leaky valves.... explosions.... everyone doesn't maintain their takes as well as I'm sure that you do.
I had the second tank filled... it was about $23
Ferrell Gas wanted about 3 times as much.... have a reputation to fail your tank so that you'll buy another.
Ferrell Gas in Sante Fe NM tried to tell me that they put 7.1 gallons of propane in my 30# tank.... it has space for 7.5 gallons but the "new" orp valve limits the 30 pound tanks to 80%... 6 gallons...that is part of the safety factor.... I will never go to a Ferrell dealer again....
No big deal... you might have to leave them over night so so that a certified technicion can do the inspection.
U-Haul dealers all over the country have propane sales.... and they are honest... I never found one than did the inspections though. They cahrge you for only what they put into your tank.
NOTE: It is my understanding that my next inspection will be due in 6 years.... more stringent.... might be time to buy new tanks... 30 pounders are about $60 each
If you have 20# tanks why don't you just trase them out at Home Depot or Walmart.... get full tanks... leave them yours... the vender would have them certified before returning them to the Trade-out market