Anyone know for sure if there is a "different" OPD for aluminum tanks vers steel ??
Yes, the valves are different for aluminum tanks when compared to steel tanks of the same size - - the "dip rod" as it is called is a different length. There was an article on the WBCCI website at one time that outlined the specific valves that were required for the aluminum tanks, but the last time I checked the article seems to have been removed from the site.
My Airstream dealer handled having my tanks refitted and re-certified as part of my Winterization last year so I didn't have to locate an outside source. If I were looking for a vendor to do the work, I would ask to be certain that they were aware of the difference between the aluminum and steel tank "dip rods", and if they weren't I would look elsewhere - - especially if they weren't willing to do the necessary research to obtain the correct part. A friend who recently retired from the LP Gas service business advised me that there is a uniform reference book that LP Gas dealers should have on-site that covers issues of valving for the aluminum and steel tanks typically used on RVs.
Refitting, re-certifying, and refilling the 20 pound tanks on my Argosy Minuet ran just short of $75.00. The 40 pound tanks on my Overlander were purchased new to replace rusty steel tanks that had been with the trailer for more than a decade.
Good luck with your project!