Drywall compound in most cases is designed for a thin application. 1/4" to 1/8". There are quick set compounds, usually have to be mixed, with water, that can be used to fill larger areas. Available from model train suppliers, one of our forum sponsors, I think??, Hydrocal. A light weight compound that I thought worked well. The Hydrocal link posted has an additional link to a U-tube video tutorial.
Link to Scenic Express catalog download. At one time there were paper catalogs, probably hard to find. The catalog, a victim of the digital age. IMO.
You may have to invest in some basic drywall/plaster tools.
Have fun 
