I've heard Dometic Penguin 2 is quieter and reliable. I am not sure why you can't run the fan all the time. I can. Set fan to high or low on thermostat then set A/C or heat as required. Fan will stay on whatever setting you choose. If you select auto fan it will do as it pleases not what you want. I have a 15k Dometic on my camper and never had an issue. It is noisy though. I also run a Supco SPP6 hard start capacitor to ease the startup of the A/C.
You can try these guys if you are thinking about a used A/C unit.
https://youngfartsrvparts.ca/ Otherwise Amazon is just about the cheapest place to get one.