You can find the line up here,
National RV Archives The models made some changes in certain years from gas to diesel within the same name.
The first thing I liked about National products was the noticable quality inside. Without even opening a door you could see the difference from other MHs. The setup of cabinets and furniture was very well thought out. The storage space inside was much better than other units we looked at. I went to the National website and read the specifications. Some of the difference are, steel framing in the walls, solid foam insulation, vacuum construction of the outter and inner walls, dual pane windows, fiberglass roof, huge basement storage with large access doors the list goes on and on. One of the other main reason we purchsed a Dolphin was the Workhorse chassis which has the GM 8.1 engine, allison transmission and 22.5" wheels.
We have owned our Dolphin 3 years and have put on 25,000 miles using it part time. We are very pleased with the Dolphin and the quality that went into the National products. I toured the factory about a year before it closed and enjoyed seeing how the mhs went together. I would recomend National to anyone .
Hope this helps.