Try this in this order:
The suction hose from the tank to the pump has a clog of some sorts in it. Unhook hose at the pump and look/blow air back towards the tank and see if it is open. Or try pouring water into the hose and see if it goes back down.
Try manually priming the pump. This can be hard to do some times pending how the pump and how it is mounted. I have been able to unhook the suction hose, power water in the hose, try and put some in the pump, hook it back up and then it takes off and pumps. Put this is a sign the end is near.
If you cannot prime it due to mounting, get a temporary piece of hose, hook to pump suction, put hose in a bucket of water about level with the pump or slight up hill and try it. It then does not have to suck up hill at least. This will tell you the pump at least does pump.
The check valves in the pump leak and will not allow the prime to start. Or the check valves have dirt in them. Or the diaphragm is shot. At this point you have exhausted the priming part and it will not pump so it has to come apart. Dirt might be the problem or the check valves are worn/damaged and will not hold. Or the diaphragm has a hole in it. They do sell rebuild kits or a new pump head if the pump is new enough. Worst case they do sell total new motor and pumps.
If you actually get it pumping and it keeps loosing prime or next time out camping will not prime, the check valves/diaphragm are getting weak and a rebuild or new pump is in order. It will drive you nuts if it keep loosing prime. If you buy a new pump or even after this eposide, get a inline pump suction strainers. Shurflo makes then, don't cost much, save a lot of heartache.
Dirt is the enemy of these pumps. If it sucks up some dirt of any kind from the tank, life is not good there after. Many/most pumps never had the suction strainer. Neither of my 2004 Sunlines ever had a strainer from the factory. They do now.
Hope this helps and good luck
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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