I'd try to do it without the relief valve, by running the permeate into a vented tank or open top PVC standpipe, then gravity flow through 4 inexpensive plastic valves that will see no pressure. You want to maximize your clean water production and backpressure on the permeate lines always reduces it.
Another interesting thing. Membranes will produce poor quality water if run at too low a pressure. You can get water out of a membrane at almost any pressure, but the water will be the highest quality when the pressure matches the membranes design pressure.
If you take your boat up a river into fresh water you'll find that the RO will produce a lot more permeate at a much lower pressure, like 300 psi. You wouldn't want to run it at 800 psi because the flux rate (permeate flow) would be way to high and you'd run the risk of fouling the membrane.
One last thing. When the differential pressure from the feed end to the discharge end starts to increase you have fouling going on. The membrane should be cleaned immediately, or you run the risk of never being able to clean it. If you clean it and the differential pressure is still high you didn't get it clean, and another immediate cleaning is warranted. Often you need to change cleaning chemistries, because the foulant may be something you weren't expecting. Membranes have a bad tendency to get into a death spiral. One bad cleaning leads to another until the membrane is shot. On a boat you are talking $100 to maybe $700. On our systems it is more like $10,000!
Gary H. Lucas