I had a tank on my RV that looked badly rusted, mainly on the bottom. I took it in and had it sandblasted and even the guy at the sandblasting shop was amazed at how much of the rust "damage" was simply cosmetic. I repainted it light gray and the tank now looks as if it was brand new.
FWIW your RV shop is either a crook or an ignoramus. There is no recertification for propane tanks only for propane cylinders. Cylinders (like for your BBQ grill) are only about 1/4 thick as that tank of yours and that is why they have to be periodically recertified. How do I know this? I researched it on the internet and now keep a copy of my research in my glove box to show the next crook/ignoramus who tries to hassle me when I purchase propane.
Steve