Ive had both an 81 and an 83 LWB that had the 30 gallon tank in them. Definitely NOT mounted behind the axle "Ramcharger style". mounted just like the 18/20 gallon tanks mount.
The particular truck I am needing a tank for is a shortbed/2wd/ so Id be "stuck with" the 18/20 gallon version anyways.
I just got an 85 LWB, have not even looked under that truck yet, to see what tank it has in it.