Inverter/Charger (modified or pure)

Remember a sales person will tell you anything they think you want to hear. A factory warranty department may tell you everything you don't want to hear. If you really want to know, call Frigidaire and tell them the refrigerator in your RV quit working and ask for some local service centers to see how they respond

Given what you want to do I would go with the full sine wave inverter.

Maybe a second sine wave inverter is in order dedicated to the fridge.
800 to 1000 watts should be fine. Jump 12 volt power off the 458 might work.

Go Power 1000w sine $425

The no frills one should work fine on MSW but you loose about 15% efficiency over running it with a PSW. If you do not boondock then this extra energy use is not a big deal. In hindsight you probably should have gone PSW when you replaced. A 500W PSW like smkettner suggested is another option.
Not sure why you are going so small when a Whirlpool 21 will come real close to fitting.

I put a Frigidaire in last July. The first day using a MSW inverter it burned out the evaporator fan circuitry. I replaced it the next day, the new fan didn't have the same circuitry and works fine on MSW. I also installed a 1000W SW inverter dedicated to the refrigerator.

The warranty is relatively useless. Frigidaire assigns a repair shop to the job & they come to look see a few days later. Once they decide the part is bad they go back to the shop & order the part from Frigidaire. A week or so later the part arrives, the shop calls to schedule the install a few days later. That schedule doesn't work when you are travelling & really never works. Even if the parts warehouse has the part the warranty requires it to be ordered from Frigidaire.

That program was unacceptable so I bought the part at Appliance Parts Depot, installed it myself and continued on my travels.

MSW inverters are nortorious for generating RFI (Radio frequency interference) this may wipe out your TELEVISION and affect Audio systems as well..

PSW inverters.. Dead silent at RF levels, NO problems with Television or Audio.

NOTE: The Radio/Audio/Television device affected does not need to be connected, Does not even need to be in your RV.

If budget allows, always go full sine. This is my rule of thumb. You won't be as limited.