What is wrong with the old unit? They can be repaired. Most shops will tell you an AC is history and needs to be replaced. Checking your unit out takes about 5 minutes. There could be a bad relay, or condenser, etc.
If you choose to replace it, you are stuck with the Penguin, as it is the only unit that Airstream has a drain pan for.
If you don't mind water running down the sides, then you can use most any unit.
However, regardless of which unit you may choose, the roof "MUST" be beefed up with 2 horizontal stringers that are attached to the fore and aft main bows, where the AC will be located.