Goodyear has a load to tire inflation chart for their ST tires. If I went to 225/75/15 goodyear tires. My 2720 weight is around 3,700 loaded, with 390 tongue weight. So the axle is carries around 3,300 divided by 2 for what the each tire carries is around 1,650 lbs.
I would get the load range D tire since it is a stronger construction and only abut $5 more at samsclub then the load range C.
Per goodyear charts attached on page 9 for the 225/75/15 tire. If it's a load range C or D, I should inflate to 35 PSI. I would add 10 psi for the service bulletin for traveling over 65 MPH. Never know when I might go over 65 mph. I would air up my tires to 45 psi.
http://www.goodyearrvtires.com/pdfs/tire-care-guide.pdf