Not that I'm aware of, at least in the systems I've looked at.
4:1 makes perfect sense if treating seawater, but quite ridiculous for fresh.
Can you adjust the cut?
Discussion
Darn. Reading up, doesn't look practical to mod the household units as the back pressure would be too high for their feeble pumps.
Those designed for marine applications can... but yeah probably not the right technology for your RV...