fuel transfer pump

Sounds like you may to be able to gravity feed it with a solenoid tap.
But then a pump may be cheaper than a tap.
Not sure of your idea of mounting fuel tanks in the rear unless it is a ute.

In my previous pajero, I had a 70l auxilary tank undeneath as well as the 135 l main tank and first used vacuum, but ended up putting a pump in for reliability.
Both tanks were about the same level.
You can get a range of bosch tanks that pump any where from 70l/hr to over 200l / hr.
I have a generic pump that does about 150 l/hr.
Getting higher pump capacities is not easy and expensive, so I would expect it to take about 45 mins to an hour to pump the fuel across.

Also, try and find a place to mount the pump that is not on the body. Some electric pumps are noisy.

Having used a diesel bladder since about '91 and having put it on the floor behind the front seats when the kids were small (they could walk on it without harming it) then later in either the rear load area or on the roof depending on the duration/terrain/load you probably won't need a pump as gravity and air pressure will shift most of it.
What I always did with the bladder was to roll it up when empty and only unroll it to use it. Put it wherever it is going to be, then fill it and when the time comes to empty it I would top up the main tank at the first opportunity and keep doing so until it was empty, then roll it up and pack it away.
I used about 2m of 1/2" hose with a plastic ball valve about a foot from the end, put the end in the filler neck of the main tank and turn it on, gravity would shift most of it and then the kids would sit/stand/squish it until it was light enough to lift out of the vehicle to drain the last little bit out.
If you really want a pump the in line plastic Whale ones commonly used for caravan water systems are rated for diesel transfer, I use one on the Humvee to pump from the auxilary to the main tank, really shifts it quickly too, about $45 from either caravan or marine supply places. A tip though, don't use the 1/2" nipple bits that push on the inlet and outlet, they split and I found it was better to just tighten a hose clamp on 1/2"hose fitted to the pump direct.

Electric sounds the go, I used both a pump and gravity and it is surprising how long it takes to drain down with gravity.

If you are interested I have 70l/hr pump that i removed from Duel tank setup in my Diesel, Only used on one trip earlier this year across the GRR. Cost me $120.00 but would let it go for about $50.00

Gujimbo,
you've got me curious about where you might be going that needs you to carry well over 200L of diesel. That would give you a range of some 2000 km. You'd be hard pressed not to pass a diesel outlet on any 2000 km stretch in Oz - but I guess I made a big assumption there (you are in Oz).