We took a northern Italian vacation with my son in a wheelchair a few years ago and are planning another one for next year. Planning and flexibility are the keys to a successful trip in Italy.
First you have to determine what are your needs and then categorize them into must haves and wants. The ADA doesn’t exist outside of America so expecting it is not realistic. For us in hotels we needed them to have a path with no stairs or steps from the front door to our room as well as a bathroom with a shower and shower chair (our preference) or a tub with hand shower and bench. For us, the toilet with rails was nice but not necessary. When we looked at places to stay, we always looked for accessibility but before we actually booked the place, we emailed the actual hotel and gave them our needs and asked if they could meet our needs and what to book on their website if they could. We found that chain hotels, including non-US chains, were often better at accommodating us than individual hotels. Unlike US hotels, we have found that a great deal of time we needed to book a more expensive room in order to have the room needed for the wheelchair.
Transportation is harder to nail down. We need to have the vehicle able to handle the wheelchair with my son in it as he needs to be lifted into the vehicles. In some places this is just an average taxi, i.e. London, but in most places it requires some special planning. We have found small tours usually have a van that serves the need with ramps into the van. In Venice we had good luck with the large boats that run up and down the grand canal and used them extensively to get around Venice. In Florence, when we happened upon an accessible taxi, we got the taxi drivers phone number and called him directly when we needed a ride. We actually paid him to take us into the surrounding villages one day for a negotiated fee which made both of us happy.
I hope this helps.