You won't tip over backwards on any launch ramp. The biggest danger you need to consider is do you have enough traction to pull the loaded trailer back out of the water. Oh and as Morsnow says, don't go so deep that the camper gets wet.
I launch boats and jet ski's from a dirt ramp all the time. I always shift into 4 wheel drive as I'm backing down and driving back up the ramp. This is more to prevent inadvertent tire slippage and gradually tearing up the ramp for the next guy than anything else though.
On a concrete ramp I use 2wd and a light foot on the throttle. If there is any tire slippage stop and put it in 4wd, drive straight up the ramp, stop at the top and shift back to 2wd before you start making any turns.
Last bit of advice assuming your truck is automatic. For pulling everything out of the water use you left foot on the brakes to keep from rolling back and your right foot on the accelerator. Give it a bit of throttle before slowly releasing the brakes. This will prevent that unnerving feeling of the whole unit rolling into the water as your right foot is moving from the brakes to the accelerator.