P0420 with surging often indicates exhaust leak affecting O2 readings. You replaced one manifold gasket - check the other side. V10s commonly have issues with both manifolds.
If total repairs exceed 50% of replacement cost with problems continuing, replacement makes sense. Refurbished Onan from reputable source with warranty is good option. Get thorough diagnosis before deciding - intermittent issues can often be isolated.
For exhaust manifold, try muffler shops or heavy truck service centers. For VacuFlush, contact marine toilet specialists or RV dealers with SeaLand experience. Both are specialty repairs.
Blown V10 plug repairable with Time-Sert or Heli-Coil thread insert. Common issue on 2-valve V10. Prevent with proper torque and anti-seize.