some/most propane sellers/fillers, will not worry about any cert if they look good and you state buying fuel for welding reasons. no rv or dot transport laws here.
but to do this do not go to a rv place. just a simple gas station, were the "kid" just does not care or is too busy.
but might be worth the little cost to get inspected. if a fail, take your fuel money elsewhere. as said a propane dealer might just do the cert if buying fill ups on two or three tanks.
my tanks new in? (two of them) 2000 or 2001, rv new in 2001. a third? as to year? but is years newer.
i have three rv tanks, (rv only holds two at a time).
must have 4-5 of the "b-b-q sized tanks.
and one little "special" unit.
i bought decades back to put in my jeep to go for a weekend of 4-wheeling. holds far more than the cheap throw away green tanks. but not as bulky as the b-b-q sized tanks. cost more than a b-b-q tank, but paid it way.
watch out on the exchange tanks, as some times they are only filled up to 3/4 - 5/8 full, but you pay for a full tank.
