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.
If Lippert no longer carries the 10540 controller, you have several options: find compatible aftermarket controllers, upgrade to newer Lippert systems, or locate refurbished units from RV salvage yards. Many newer Lippert controllers are backward compatible.
The front run bay control panel #166-2011 for your 1999 Monaco LaPalma is a discontinued OEM part, but several sources still carry new and refurbished units. You'll need to verify compatibility with your F53 chassis electrical system before installation.