The large fuel tanker has two conditions:
- the airplane refueling point must be higher than 3.40 meters from ground
AND
- the total fuel capacity must be > 3500 gallons for the model (still a large one) with a shorter tanker, and > 8000 for the largest one.
If your airplane meets both conditions, one of the two versions of the fuel tanker will arrive. If the plane has a non-standard fuel system, it's *possible* it might not report the total fuel capacity correctly, maybe that's the issue, assuming the refueling point under the wing it's high enough.
The smaller truck will not use the fuel position you entered, so it will always stop at the side of the airplane, with the side used depending on the fuel position (left if negative, right if positive)