Won't start I get a single loud click when I turn the key. I replaced the starter and it worked for a day, intermitantly. I replaced the relay but no change, I just get a single loud click. What do I do next?

Is this a loud click or a quiet click? Is it a single click or lots of fast repeating clicks? A quiet single click is most likely the starter relay in the fuse box failing. A louder single click is related to the starter motor, the starter solenoid or related parts. A repeating, rapid clicking is usually a power supply issue. Is there any warning lights on the dashboard? Were you having any other issues before this happened?
Try going back to basics. You may have already tried some of these.
Check the battery is the correct one for your vehicle and is fully charged. A V8 engine takes a lot of effort to physically turn over and a weak or wrong battery will struggle to turn the engine over. Check all the connections on the battery are clean and secure. Check the other end of each battery cable is also clean and secure. A bad electrical connection can drain the battery so it doesn't have enough power for the starter.
Apply the park brake and put the vehicle in Neutral. Try starting the vehicle. This disconnects the transmission making it easier for the engine to turn over. Does that make any difference? Comment below with more information and I'll try to help you more.

Is it a single click or a repeating click? A single click could be an ignition issue at the key end. A multiple click could be a wiring issue to the starter. If these aren't the problem you could be looking at mechanical issues inside the engine and gearbox.

Check battery connections and battery fuse. Battery may not be large enough for the size of your van

Is the battery at 12.6 volts,old or dead?

Cables loose?

Starter Solenoid bad or starter bad

THAT'S WHAT I THINK IT IS

If i understand you clearly. your van needs to be checked for mobiliser operation and/or key sensor ( Try spere key)(my email add: [email protected])

when key is turned there is a single click and then nothing happens. Where do you hear this click coming from ? Before you replace parts you should test the electrical circuits , if you don't know how i suggest you take it to a qualified repair shop .
Viewing wiring diagrams to see what all is involved in the starting of the vehicle , reading how it's suppose to work in factory service repair manual .
Moving the ignition switch to the START position sends a 12-volt signal to the engine control module (ECM) to command starting. Battery voltage also travels through the switch side of the Run/Crank relay then through the clutch switch, if equipped with manual transmission, and on through the coil side of the Starter relay and then to the ECM. The ECM receives class 2 information from the vehicle theft deterrent (VTD) and transmission control module (TCM) to verify that all parameters are met for starting. When all parameters are met the ECM grounds the control circuit of the Starter relay closing the switch providing battery voltage to the starter solenoid through a 30 A Maxi-Fuse.

Are the battery cables good ? Voltage drop testing

Starter Voltage Drop

Test the control side of the circuit at the relay . Free wiring diagrams here http://www.bbbind.com/free-tsb Enter vehicle info. year , make , model an engine size . Under system click on engine .Then under subsystem click on starting . Click the search button then the blue link . Pretty complex looking isn't it . Using a factory scan tool at the dealer makes diagnosing a lot simpler . Can view data to the PCM , start signal from the ignition switch , Starter relay control , see input from transmission park neutral safety switch .

Starter motor relay! - not sure which one it is - definitely sounds like starter relay tho.
if you disconnect the main wire from the starter and turn the key you will kear the relay clicking, find it and replace it... should solve your problem.

Bad/malfunctioning relay. Let me know if you want to work on this.

A little clarification: the starter relay and starter solenoid are the same piece of equipment -- just different names for the same device.

Charlie