I don't understand what made you drop the tank to replace the pump? Is your pump not inside the van?
I guess the 0.5 bar PRV that you refer to is required to prevent pressurising the tank when filling from the mains?
Mine definitely has some sort of PRV as I can hear it vibrating like hell when the tank is full and water is being relieved from under the van.
Is there some sort of simple reed valve device which would prevent crap getting in the tank, but allow air out during the fill? This would be simpler?