[Eleonora]

Hello @bamagirl – first the “unofficial” answer: if you have a visibile disability, i.e. you arrive at the museum on a wheelchair, they usually will not even ask for proof – it is a question of practical sense, they see a person with a disability, it is clear, no need for proof and they won’t hassle you with documents. But: this is my experience with friends with a visible disability (a mobility one in the specific – wheelchair user).
In Europe there is going to be now – in a few years, it was approved by the EU institutions, now they will have to implement it in the various countries – an EU disability card, for EU citizens, which the person just shows and gets the free admissions/reductions etc – but if you are coming from the US, my suggestion is to show any document you have with “proof” of disability – and do not worry about the percentages, it should not be an issue. Just explain that this is the document that they give in the US and they should give you the various reductions/free entrances.
I did a quick check on the Municipal museums website and they do not specify which documents are needed, so this means it should be ok with any document. Enjoy Italy!