Slow cranking, weak battery, battery discharges after van sits awhile

Check your battery cables. The battery cables could be routed by engine or exhaust components that get the cables hot which in turn will cause resistance and cause the slow cranking. The fact that it starts fine after sitting tells me something is probably getting heat soaked. If you're sure it's not the starter I would check the cables and especially ground connections to the frame, engine block, and body. Check resistance with a multimeter if you are able. If they are the original cables I bet that's it. If the starter and alternator are good then the cables are probably at fault. I would also check the starter relay but that wouldn't cause slow cranking..

Check and clean, tighten all grounds .

have battery and alternator tested most auto stores offer this service for free

SOUNDS LIKE BATTERY IS WEAK,,TAKE IT TO AUTOZONE HAVE BATTERY CHECKED!

Check body control module and wiring at the starter motor.

could have a battery ground , check cables for corrosion.
could also be the starter or the solenoid.
you could also have a dead short in the battery itself, it will be ok if it sits but will give you fits for repeated starts

best guess is you need to replace you crank shaft sensor, it is located on the bach of the engine ( fire wall side ) just above the oil pan rail close to the bell housing. when the engine is cold the sensor will have a stronger signal but as it heats up the sensor will fade so when the engine is already at operating tamp. and you turn the engine off it actually gets hotter because the water pump is not moving the coolant any more this is why the sensor loses it signal, the sensor gets weak slowly over time what you will notice after replacing the sensor it the engine will perform much better then it has for quite awhile.

Try turning the key on (but don't crank the engine) and wait about 2 minutes, then try and start the van. Anytime mine sits more than overnight, this is the only way it will start, otherwise runs fine.

Hope this helps.

You can tell if the battery is weak while it is cranking. If that is the case then battery need to recharged. When a car is not running regularly, it doesn't have the charge from the alternator so it becomes weak. If there is no sign of weakness in cranking then the problem is not from the battery, but it is from lacking of two basic things: ignition and / or fuel. Need to find out what is the root cause. Good luck.

you may very well be on the right track here... if it were me and my van here, i would replace the dimmer. i bet you can get another from a parts house without going to the dealer. then again... get the price from the parts house and see if the dealer will match the price for the part ( under the current big 3 situation they may do it)