They were distinctly different models with different quality and weight levels. Hi-Lo was the original model with the highest quality materials and trim, it eventually became known as the "Classic" model. The Funlite was lighter-weight model that came out in about 1987. The Towlight came out in 1992 and it was the lightest and lowest priced model. It had a wood framework initially but eventually replaced it with an aluminum framework in the walls. The Funlite was phased out after the Towlite came into its own.
Here is a link to the brochures that Hi-Lo had over the years. I'm sure you will find it interesting. The brochures are in PDF format.