I have a 1976 Dodge B300 (1ton van) based class C RV. It has a 360cc V8 with a 2 barrel Holly carb. 89k miles. When I try to measure manifold vacuum the gauge needle bounces to below 0 (positive pressure) in time with the engine. I'm convinced I have one bad cylinder.  I have one fouled spark plug. 

The advice I need is about what I can do myself to further pinpoint the specific problem. I am a disabled veteran living on a VA pension. So, money is very tight. The RV IS my home.  I am not a mechanic. Though as an electronics technician of 45 years, I'm not afraid of servicing my own stuff. I'd like to know exactly what needs fixed or replaced before I try to find a mechanic willing to work on my old carborated engine.  Or better still, if I can fix it myself! Thanks in advance for any advice.