Pull the fw onto enough boards to make it level. Then measure the rail clearance. Just say it's 3 inches and you want 7, you then know that you need to lower pin and or raise the hitch 4 in and lower the trailer suspension 4 in plus the thickness of the lumber under the tires.
There are a couple tricks that will raise the rv (lower the suspension)) a small amount but if you're that close to the rail you are more likely looking at an axle flip or sub frame.
If you are looking for lift of the RV it may also be the time to look at things like 16 inch tires vs 15. It doesn't raise the rig much but it can help.