Assuming it's a Ford E-SuperDuty chassis (which was pretty soon renamed to the E-450), you do have a 55 gallon gas tank. Probably also if it's the E-350 chassis.
Sorry to hear about your misery. I wonder if the transmission fluid was low and/or needed changing to begin with? I believe on these Ford's it's supposed to be drained and replaced every 30K miles. (There is a drain on the torque converter, so it's possible to do a fairly complete fluid change by gravitation alone.)
If you haven't already done so and don't know when it was last done, I would suggest changing the brake fluid and possibly the flexible brake lines as well. Brake fluid tends to absorb moisture from the air over time, and that moisture lowers its boiling point and can cause corrosion of brake system components. Boiling brake fluid means basically no braking ability...not good in the mountains.