Having owned four Saint Bernards I am very familiar with your issue. Dogs with floppy ears are prone to infections as the ears do not dry well. In order to keep my Saints from the infections ( after learning from many re-occuring infections) , I dry their ears with leftover cotton batting. I dothis on a fairly regular basis or when ever they have gotten them wet , particulary after a bath or swimming. There is also a salve that is a sulfur base that I used periodically , squirting about 1 tsp into the ear canal and massaging it in.
I also notice a tendancy to have ear wax build up. I can usually tell because they "flip/flop" their ears back and forth a bit more than normal. Keeping the ear wax out is important as it traps the moisture in the ears and promotes bacteria growth. If the wax issue is a constant they sell a ear cleaner ( its blue ) at any pet supply , I squirt in a few table spoons and then massage to break up any build up and remove by using cotton batting scraps , working the batting into the crevises.
Once I really caught on about the water in the ears and periodically cleaning them I was able to keep the infections at a minium. If I senced there was a bigger problem on the verge ... I used the sulfer ointment a few times in a day and problem averted.
Oh one other thing .. when ever they are sleeping I flip up their ears so they can "breathe" that really seems to help alot.
So .. keep the ears as dry as possible.. bacteria just love the moisture. I know Newfies just love the water!
I was so greatful I had a vet that had four Saint Bernards ( yes at one time) of his own and was very familar with giant floppy ear dogs and their issues.
Hugs to your big fuzzy one!!