From what I can see from here, I do not see an issue with your approach. I'm assuming something will be done on the inside as well. The curved ends on your template, do not know if this was intentional or not. If you meant it to be that way, I do not see it hurting anything however do not know if will help any either.
Have not heard the term "rosetta" used like this before. However if they are referring to punching/drill or cutting a few select holes in the reinforcement plate to then plug weld or weld around the ID of the hole to join it to the tube to help spread the load, yes this is done often when laminating thinner plates to make a heavier one.
As to the 1/8" tube A frame, I know what your welder is referring too. They keep light-weighting the campers constantly and you end up with what you have. If they are use to dealing with equipment trailers and they see an RV... their head will shake from the time it comes in until the time it leaves...
Good luck and let us know how it goes.
John