Go back to the dealer and see what they suggest as a solution. They should have worked with you to make sure your truck and trailer were setup properly before letting you off of the lot.
Some 5th wheels have adjustements possible to raise or lower the axle(s) at the 5th wheel spring shackles. That would be your best bet.
Otherwise you may need to do what is called an axle flip where the axles ride on the bottom of the springs or blocks added to raise the trailer if the axles are already on the bottom.
Your RV dealer is the best resource.