RTI = "Ramp Travel Index."
I don't have any plans handy, but I'm wanting to think that all it is is a sturty ramp with a 20-degree slope, about 4-5' high at the high end.
If you paid attention in middle school Trig, you should be able to work out plans.
Design load should be about two tonnes - which is a bit of overkill, but should keep the thing from collapsing under the weight of a modified rig. If you plan to RTI full-size trucks, then design for a working load of about 7000#. Overkill in engineering is not a bad thing, properly applied and used...
If you Google RTI ramps, you should be able to confirm that angle, then apply some basic trig (damn thing's a right triangle anyhow...) to get your measurements.
5-90
(Just did a little checking of my own - a typical RTI ramp is 20*, and a "high-flex vehicle" RTI ramp is 23*. Break out your trig tables to get the rest of what you need - I've given you enough information already...)