You can do that, but if you go too far you will find the transmission downshifting to get the revs back up where they need to be, to have the horsepower for the speed you are trying to run.
You won't know how much unless you have the tools and information (engine's power curve, power vs. speed curve for the RV) to do the chassis engineering. The P-30 chassis series came with different rear end gears (in some cases different axles) for different GVWR/GCWRs and applications (RV vs delivery van, etc). You should be able to find a different ratio for a GCWR downgrade.
A couple of vendors offer add-on overdrive/underdrive systems, and in some cases know what they are doing.