I've been down this road, except my tank was rusted pretty bad. It was right behind the front wheel and my coach had been subjected to road salt out of IN for 9 years. I ended up replacing it as I'm not a very good painter and couldn't afford the downtime for someone else to do it. $500 for the tank, and 95% of RV tanks are made by Mansel Tank in IN.
Here's the smoking gun:
www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/smalllpgas-chapt3.pdf
It describes ASME tank construction perfectly for what we have in our Class A's. It indicates no re-inspection according to this document. It doesn't have an official US DOT or PHMSA seal/logo/whatever but that's good enough for me. I got it by doing a search for 'propane' at US DOT Pipeline, Hazardous Materials and Safety Administration's website:
http://phmsa.dot.gov/hazmat/regs/sp-a/approvals/cylinders
Here's the link on portable cylinders:
www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Files/Hazmat/requal_propane_cylinders.pdf
I have both copies printed and in my coach's briefcase. Letting your Flyin' J operator read something "official" will get you some LP with less hassle.
Best,
- bob