96 chevy flair - onan marquis 5000

The current gas gensets from Onan will easily fit the space where your older Marquis was fitted. Length and width same, newer gensets about 1 inch taller. All the hook-ups are on left side of new sets, so you may have to lengthen batt cables, fuel line, a.c. wire harness etc. Onan offers 5500 and 7000 watt models; Model 5.5HGJAB, and 7HGJAB. Onan also has EFI units, same size dimensions, but higher priced, and will have to install high pressure fuel pump for EFI units.
I have done this swap numerous times, pretty easy. The newer sets are quieter, as they have full sound enclosure, but I really like older sets if they are repairable, as they run at lower speed, have bigger oil sump, and gen is direct drive from engine, versus belt drive on newer sets.

One like this from Harbor Freight is the best bang for the buck.
Predator genset
Need twist lock adapter as required for must gensets

Where it seems like my only option is fixing the existing one. I am thinking of getting a temporary generator to run on while I fix the existing and possibly hang on to as a future back up. My thoughts are getting one with has the 30 amp plug that fits my RV and at least 2k watt output. I am assuming my onan does 5k watts do to the name.

I didn't say I was ready to give up on her yet. Just curious about replacement options. I am willing to fix it and I would be fixing it myself. It would just suck if I put in the work just to find out it has no output or if more stuff just keeps breaking down the line. The previous owner believed it was the carburetor when it didn't start. The other things that concern me is the level of rust. I also know the previous owner and he doesn't take good care of his stuff. Even if I didn't know him it's apparent by the condition of the RV. If my only option for replacements is onan it would appear they start at 3k and just go up in price from there. If that's true then replacement doesn't seem like a viable option at all.

Make sure you have the proper oil level in the generator and over 1/4 tank of gas.
Try changing the fuel filter. Do the simple things first.
If it starts but stops quickly it means the generator runs. You just have to get it to stay running.
There should be an hour meter for the generator somewhere, usually on the dashboard.
Don't be too quick to give up on your Onan.

Spending a couple hundred dollars on your existing genset would be far cheaper then $5000-6000.
Pull a plug wire off, remove spark plug. Set the plug on good metal portion of genset with plug wire connected. Crank over engine and watch for plug to spark. If it does, likely a fuel issue.
Remove fuel line from the genset. Apply some low air pressure towards the fuel tank, listen for gas to bubble. Could have bad fuel line. Easy on most to replace.

What, besides fuel problems (which are common for generators that have sat), leads you to believe the generator is in poor condition? Replacing the fuel pump, fuel lines, and maybe carburetor will be far more economical than replacing the generator. The fuel pump in particular can be replaced inexpensively; it's a pretty standard low pressure fuel pump, such as is used on older cars, farm equipment, etc.

(Incidentally, if the engine cranks, it's by definition turning over...though not necessarily starting and running.)

Cranks but won't turn over. If it does start it stalls out fast. The fuel pump has no pull. So I'm looking at a fuel pump, fuel filter, maybe some line replacement, and possibly carb rebuild. That's just to start. I recently bought the RV. I don't know how many hours. It's 24/25 years old. So before I commit I want to research options for replacements. Do to the poor condition of the generator there very well might be more problems waiting around the corner.

Onan Marquis 5000!
What is wrong with existing genset?
How many hours on unit?