Welcome to the forum. Look in the library for the 1990 25ft. Classic manual. Most years are the same and the manuals aren't always that detailed. Ask us specific questions and we will do our best to answer them. Many of the members have older trailers and we can repair just about anything.
The manual only covers the trailer itself. The appliances,such as, the fridge, furnace, stove, power converter, water pump, water heater, etc. have their own manuals. Some can be found in the reference library and many can be found online by Google search or the companies themselves. Put in the name and model number and you may find the manual.
Try this 1981 manual, there is very little change from year to year. One manual blankets all trailer sizes. For the appliances, go to the manufacturers' websites and search for the manuals. http://www.hilotrailerforum.com/f49/1981-hilo-models-3467/