Lifter noise vs cracked manifold noise

Thanks for all your responses. I visited my RV today to fetch a few things and while there I took a look at the engine compartment. I didn't run it at all because I didn't have any hose with me. After looking at the engine it looks as if whatever it is it'll be a PITA for me to fix it. However, a passenger side exhaust manifold issue looks like the easiest fix. I also need to replace my oil pressure gauge as the oil pressure was erratic during my trip. The gauge is not marked with any meaningful scale, only rather is marked Low to High. It never drops below low but will run between 1/4 to 3/4's gauge during similar running conditions. For instance, I can cruise at 65mph on flat terrain and the gauge will vary from 1/4 to 3/4's with no change in speed or temperature. Very strange, maybe it's a Ford thing...