Here is an example of a 'saddle' tank of the era Bette Cox is looking for.

You can see the tank situated behind the cab. There is an identical tank on the other side and they are connected by a structure that is sort of like a platform. It 'hangs' over the frame like a saddle might. It can be filled from either side.

Some of the trucks of today have 'side saddle' tanks. They are just tanks on the frame on either side of the truck.

Hope this helps.

Cheers, George
http://www.hankstruckpictures.com/pix/trucks/martin_phippard/mack/file0032.jpg