Well my first thoughts were based on one of Dingos earlier write ups....
A clean drum of clean diesel is a good way to rule out a bunch of problems at the tank end but it is kind of jury rigged, did you use clear hoses into the jerry can so you could check they weren't drawing air when going uphill?
How does the clear section to the fuel pump look? mine is kind of yellowed with age and it takes some looking to see bubbles going through it, I ask because the fuel filter is the next place I would be looking. Change it for a new one of a decent make (Mann or Hengst or Merc but that will be either Mann or Hengst with a MB badge on from what I can tell) and be careful to use the new o-rings that come with it and take care not to damage them refitting it. The pipes in and out of the filter are decidedly dicey, the metal retainer does not actually clamp them into the filter housing so they depend on the plastic end fittings being intact (check for damage) and the o-rings. If someone has changed the o-rings for inferior ones (good o-rings are supplied individually wrapped and lightly lubricated, dry o-rings out of bulk boxes are unreliable particularly in diesel lines) it may just be these (merc dealer can supply).
You are correct about there being no lift pump on these, the distributor pump does all the work and it doesn't like sucking just air (as Dingo says it will self bleed air bubbles from fuel) and uses the leak off fuel for lubrication so running it dry can damage the seals; however I would expect seal damage to affect it all the time not just on hills. If you end up with a section of pipe with no fuel in at all it will take a good few seconds of cranking to get the van started.
You still didn't really explain if the revs drop over a few seconds or if it just cuts out like flicking a switch. I would also have to wonder if there is anything loose in the engine bay near the pump that could move when you hit a steep hill, and/or of the nut that holds the wire on the fuel solenoid is loose. I'm basically wondering if you could be getting a short (or disconnect) which causes the solenoid to close temporarily.
In fact, thinking about completing the circuit, crawl under the passenger side (I would start behind the front wheel but looking forward) and look for where the gearbox bellhousing is bolted to the back of the engine block. At one of the top bolts sort of behind the oil cooler, there is a braided earth wire which runs across to a stud on the chassis box girder. Have a look at the state of this braided wire, does it look frayed or strained? The engine and gearbox mounts have rubber blocks to damp vibration, it is possible that climbing steep hills they move backwards slightly and strain this wire - if some of the braids are damaged you will be getting a poor earth from the engine back through the chassis to the battery, it might not completely break the circuit, but it just has to increase resistance enough that the solenoid is not held open (or some other engine electronics fails). You can get replacement earth braids in Halfords, it does no harm to double them up just to be sure.
Good luck
Jim