I fill my rib right up with gear then tie a tarp over the top, I carry all the camping gear, gas stove,kids bikes,2 fuel gerry cans, 3 20 lt water drums etc.if a ferry crossing doesn't allow fuel in cans fill them once you've crossed the ferry close to your destination.
The fridge, computer, cameras, fishing gear, clothes, dog, 3 kids and Mrs all go in the 4x4. Last boat trip for us was 7000 km, the one before was 14500 km much of which was off-road.
My trailer is a single axle heavy duty, the boat is strapped down as tight as possible to stop any bouncing on the rollers.
I normally replace and grease and replace trailer bearings before each big trip with quality bearings. Check your bearings to make sure they aren't getting hot during the trip, the last thing you need is a wheel coming off especially on a loaded outfit. Keep an eye on tyre pressure on both the trailer and tow vehicle.