I'm having trouble picturing 6 foot tall cabinet doors above a window on a travel trailer. :h Is this an interior or exterior cabinet? How tall is the RV? Do you mean 6 inch tall doors?
Gas shock springs come in all different sizes and strengths. You should be able to find one that is workable without too much effort; maybe adapting one from a junkyard hatchback or SUV or minivan would be workable if this is indeed a six foot door.
The vertically opening cabinets on my Coachmen motorhome have little spring cylinders to keep them open. The cabinet doors are 14" tall, and the springs roughly 6" long extended.
These look rather similar, with apparently better designed mounting brackets. This is not a particularly exotic piece of cabinet hardware; several variations are available.

