I think you'll *probably* be ok down Main beach. There are a handful of exits (off beach to camp sites) which are doable provided you hit them at low tide and can therefore get a good runup and heaps of momentum. There should be plenty of space down there. I can't say for sure about Flinders - it will obviously be busier but not sure how busy. At least with that many people around there would be someone to help you.
I took my camper trailer to straddie a couple of months ago without any real problems but it is only about 1.1t and I have a turbo-diesel 100 series. It did take me a couple of goes to get up the exists - but easy to back down and try again with more right boot.
I was fishing there on Sunday and we struggled to find fish but eventually found some tailor. Apparently (reading another report on here) they were biting down near the pin - I wish I had gone down that far!