As Quincy said look for the water.
A hissing sound is interesting if we are talking about a water leak. Are you hooked to city water? If not, does the water pump run when the hissing is present? I'm not familiar with your coach. Does it have propane? Is this a kitchen slide?
Sorry for questions with no answers.
Need a little more info to help find a solution.
Good luck and all the best!