I don't know specifically what model you have, but in the 2000-2001 timeframe Beaver is known to have undersized the hydraulic hoses (either by accident or poor procurement) for the main slide. The pressure rating should be at least 5,000 psi but the hoses used were 2,500 psi. Many of us have had those replaced. I had mine done by a truck service center in Sioux Falls. The lines themselves are several hundred dollars because 6,000 psi line isn't cheap and the labor consists of threading the new lines along the frame rails. Most of us have left the old lines in place after flushing or capping them. It cost me ~$1000 three years ago and most of a day.
As for keeping the slide in, I don't have a lot of encouragement. If yours is a Magnum chassis Beaver, the main slide uses a double-sided cylinder so one side is always under pressure, either to keep it in or out. With one side unpressurized there is no easy way to keep it in. I could barely go around a corner before I needed to shut everything down so I could bring the slide in again. I honestly don't think you need to go home to a known mechanic; this isn't rocket science.
Good luck, and BTW, be sure to replace both hoses even though the leak is only in one of them.