[Little]

As a lifetime Cummins mechanic and warranty rep I too would nix the 5.9. You can easily get the power you need but the torque wont be near what is capable from the 8.3.
Unfortunately the engine you have is probably the worst engine Cummins produced, so whatever you replace it with will be an improvement. You may need to look at an overdrive trans, as the 8.3 runs best at under 1800 rpm.
8.3 is one of the most reliable, fuel efficient engines built. I have a farm tractor (Versatile 555) that we swapped out the 555 with an 8.3.
There should be some scrap stick/staples Rv's with good 8.3 mechanical powertrains. Lots will have under 50k miles. Good luck, you should have the perfect rig when finished.