You probably already do have engine heat to your hydronic system, Ron, though I'm not familiar with your model year. As such, I'm no expert, but I don't know of any such switch; the engine heat is automatically exchanged as needed to the hydronic system (Hurricane or Aqua/HydroHot furnace & hot water).
Many confuse that idea with the option to flip a switch sometime before starting the engine to depart camp in cold weather, thus pre-heating the engine by circulating hydronic heat to it. A preheated engine has to work less to start. In that case, there is indeed such a switch, usually located with or near the other hydronic system on/off switches. It should be labeled "Engine Preheat" or something similar.
I'm sure others here with 1998 Patriots will help zero you in on the Preheat switch location, but I don't think you have (or need) one to have the engine help heat the coach.
-Joel