Seems like the problem is that the ramp is too shallow, not your van or the trailer. I doubt an extra 12" of tongue would make a difference.
**That's what she said!**
Is there another ramp you could launch at? Our Mastercrafts have to sit up higher because you can't lift the prop out of the water like a outboard lower unit can.
At least if it is that shallow, the water is not over your door sills when you back in.