Changed the pump and that didn't improve things so I gather...... PS: Before doing the stuff below, remove line from bottom of hand pump and blow down it, have someone outside to listen, you should hear the water gurgling, connect pump back up and see if it works, if ot does then you have mor ethan likely got some plastic drill swirl in the tank blocking the hole...........
If your system has 2 tanks I would be heading for the 3 way valve, remove it and replace with a straight joiner so one tank is online then see how pump works, if the pump operates properly then the 3 way is a dud and can be replaced cheaply with 2 Pope garden irrigation taps and T piece, Bunnings all up about $9 ............. Only one tank I would start by checking the water line to the tank making sure there are no kinks in the line, any sort of crease could close up when suction applied, checking all joins closely, next I would pull off the inline filter and make sure it is clean with good connections, then assuming all good so far, remove the hose form the tank and see if it flows freely............ thats assuming the new pump is failing the same as the old pump, the water running back to tank is a sure sign of it loosing suction so it does point to a bad join/connection somewhere, air will enter where water won't leak, maybe giving the pump a lube of vaseline is in order as well......
Have a look at the ends of the hose at joins any splitting, hard bits or bulging cut the bit off...
If it is still playing up I would pull out the pump, run a new line from tank, spare bit of garden hose and hook it up to the hand pump,doing it outside the van is easier and see if it operates, if it does then you know for sure its not the pump.. If the pump works fine then just take the easy way and replace the water line with some drinking quality hose and new hose clamps......
With Big Mal I had a suction issue, ginned around testing sections, after an hour or so and still no good, I ripped out the line and replaced it with a bit I cut from one of the vans water hoses I carry, 30 mins and all fixed, so much easier..... I now have a nice short length hose which comes in handy at times......
