We had the exact same experience last September. DS was just really "potty trained" but still had to be reminded a million times a day. We never used a topper in the parks so I can't offer any advice to that (I find it easier to teach him to turn sideways and sit longways across the potty - it's weird but it keeps his little tushy from falling in and prevents me from hauling around a topper. Of course, we cover the potty up with sanicovers so he's not draped over a public toliet seat) We did use a topper at the hotel.
The best advice I can offer is to make sure you are constantly asking if s/he needs to go potty. We tried to get in the habit of before and after every meal, but it's hard to remember sometimes. Toddlers can get very overwhelmed at Disney with all the excitement and then of course the forget to go
We brought 2 changes of clothes in the park in clear ziploc bags with an extra ziploc packed within for wet clothes. It was helpful for an accident or if he simply wanted to play in water and needed spare clothes. We only had 1 accident and it was simply because of what I mentioned above - he got excited and enthralled in Mickey's House and forgot to tell us he needed to go. We, also in our excitement, had neglected to ask him after lunch to go potty.
So, he peed. In Mickey's House
A giant, 2 ft. in diameter pee puddle right in Mickey's living room.
We were super impressed with Disney's handling of the situation too. DH went to find someone to alert them *ahem* of the mess, and the first question they asked was if he was okay and did he need a change of clothes. Even for the Mouse, I was impressed with the speed and efficiency with which they handled things.
I would just assume you will likely have an accident at some point and be prepared.