Hi Sylvie:- Northern lights is primarily a marine diesel unit. I don't know how they have added the rad but make sure that the unit you buy has a rad, because marine units use the lake or ocean water to cool the engine.?
Gensets run at different RPMs depending on their design. Skid mounted diesel units mostly run at 1800 RPM. The engines are designed for that and they get a very long life from the engine, typically more that 30,000 hours. These units are used mostly for remote power applications and high end RVs. Most RV genset especially gas or propane units run at 3600RPMs. You can use a smaller engine to get the same KW. (higher rpm, more power).
Portable gensets (not mounted RV types) such as Honda or Yamaha are going more and more toward inverter technology so they operate at variable speed. This saves on fuel and engine wear for applications when not a lot of power is being drawn from the genset.
As some of the other posters have said -- Stay with the brand names. Onan has service all over the continent, even up into Alaska and all along that route. Honda and Yamaha as well.
Onan makes gensets for RVs of all types even the 1800RPM Diesels, they will be more money than the Northern lights but not much more. Also remember what I said about the rad. Depending on where you are buying from the unit might need to have a rad installed. Buying a proper RV genset will prove to be better in the long run.
Cheers
Chris