How heavy is your trailer? It's a triple axle so there must be A LOT of weight in there. You might be going way over the van factory tow rating and I think the max is about 10,000 lbs with the right gearing and engine combo. Be careful if you are.
As far as the injectors are concerned, if you're doing 1, do them all.
The 6.0's had issues with casting sand being left in the block so add a coolant filter.
Gauges are a must. Synthetic oil is a must.
As for the rest of it, if you're going to be towing super heavy on a regular basis, I'd upgrade the gears before I'd add a tune. That'll do a lot more to ease towing stress than a tune will.
Then when towing, watch your temperatures. If they start creeping up into the danger zones, slow down. If you have to, pull over for 5-10 minutes but keep the motor running so the fluids circulate as they cool down.
You can also add a manual fan control switch and turn your fan on earlier than the computer will.
2005 Ford E-350 Diesel Sportsmobile
EB 102 Voyager top, 3.55 rear, Mobil 1 all around
270 amp alternator, billet water pump, coolant filter kit,
aluminum radiator, X-Monitor with pyro, trans temp, boost
+ Scan Gauge & SCT Live Wire (Economy)