Water pressure drop

Hi everyone first post so hope it’s in the right place. So I have a 2014 Mercedes spring class c and I’m having water pressure drop problems running off the pump but it’s not consistent. I got a new pump even though I check and is was pumping 3 Gpm when I removed it from the camper and bench tested it. I put it back in and still wouldn’t work just build pressure and instantly drop. So then I tried disconnecting the system from the tank and drew water from a bucket and it worked fine. So I hooked it back up to the tank thinking maybe the pump wasn’t self priming like it says it should. And after I hooked it back up it work with the rv tank as a supply. I ran the water for 7 minutes with not drop in pressure. I thought the problem was fix so I took it out this weekend and it quiet working again. I have tried a few other things and sometimes it will work and then it doesn’t very frustrating. So hopefully I’m missing something that someone else has had the same problem and can help cause I have to have it fix by this next weekend. Thanks for any advice.