Is the pump running but not pumping water, or not running?
A few things to check: water in the tank... , selector is pointing to a tank inlet(not both at once), check any filters in the line, remove tank filler cap while listening for a vacuum (possible blocked vent pipe)
Pump is running but not pumping water. Tanks have water in them but the selector is drawing from both tanks I believe... I'm not that technically minded but i pulled the pipes off the pump and set up a hose in a bucket and the pump wouldn't pump it out of the bucket.
If you are handy, the shurflo is quite easy to service. It has about 6 or 8 screws (the big ones) around the edges of the black housing that can be removed to inspect the diaphragm for damage. Carefully remember how it goes together as you separate the 2 halves.
You can buy all the internal bits
The housing that comes away also contains a non return valve under the pressure sensor (the 4 screws holding the bit with the wires going in) so check it opens when you press. It also has a small bypass valve (another few screws) but this in unlikely to be the problem (and tricky to adjust)
Sounds more like a blockage or an air lock. You can prime the hand pump by putting the spout in a jug of water. There are probably also one-way valves in the water line. They can cause problems. You may need to start from the water tanks, pulling pipes off to ensure you have water flow.
Beware: Start of waffle....
These types of problems can be a PITA. Is it one common problem, is it two separate problems, did messing around with the first problem cause the second one? Worse still is when messing around appears to have fixed the problem, but it comes back again.
We all usually start by trying to find the quick 5-minute fix. When that fails, reality sets in and we need to step back, grab a coffee, and settle in for a pleasant afternoon of slow, meticulous, step by step fault-finding. At least at the end of it you will know the entire water supply system of your van. That will make fault finding much easier in the future.
... End of waffle
Two tanks connected together to a single outlet can be a pain. There can be situations were the pumps draw air even though the tanks have water.
Forget about hand pump,in fact isolate it if possible .............. draw from only one tank at a time............ run a direct line from a tank to pump, if possible let flow from pump just run free, this will show if pump is working or stuffed, if pump is working but won't when connected to normal plumbing then you have an air leak, more than likely the tank selector valve........................ go for the simple easy things first, the pump is usually not at fault, dodgy selector or loose hose fittings are the most common culprits............ hand pumps are rubbish and can actually cause the system to fail as they allow air to be drawn in, depends on the plumbing but it could be the cause of your problems............ you can use a bit of garden hose or just some clear plastic hose from Bunnings, works well as water line.............. if you dismantle the hand pump give it a squirt of olive oil it may start working then........
Checked everything out and there was some air in the hand pump line so flushed it.... water pump ended up being cactus so replaced it and now it works perfectly! Thanks for all your help couldn't have done it otherwise.