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My experience has been very positive using an axeon hf5 membrane at 50 psi tap water pressure. My membrane is over 10 years old and still producing at 97% rejection rate. Our tap water TDs is around 130ppm. When processing water my waste to pure ratio is around 55 waste to 45 pure.

I have never heard of the membrane's warranty being affected by using a lower water pressure than 80psi. That's a new one on me.

Be careful about suppliers claiming to sell a low pressure membrane. One supplier advertisers a low pressure membrane, but the specs show the membrane needs 125psi of water pressure applied to it.

A blue water sailing boat with a water maker on board needs a 600psi water pressure applied to the membrane. In comparison, 125psi is low pressure I guess.

Yes the price of an axeon  hf5 membrane is crazy, but they are the only true low pressure membrane on the market.

When using an onboard r/o unit, most suppliers supply a 4040. At 50 psi I get roughly 2 litres of pure every minute. A 4021 will do half that.

If going the onboard route I would recommend fitting a float switch to cut water to the r/o when the tank is full. Purefreedom fit a float switch as standard on their onboard r/o systems. It saves flooding your van out.

I also recommend fitting 2 x 20" prefilters. The first one is a sediment filter and I recommend you go for one with a clear bowl so you can monitor the state of the sediment filter visually.