Or you could just use a standard 3 way light switch and select your power source manually. Wire the outlet to the common terminal on the switch, normal power feed to one terminal and invertor feed to the other. Be careful to choose an inverter that will meet the power quality requirements of the cpap.
...How do we get a refund?
What does your contract say? Every right you have is in the contract. If it isn't in the contract then it doesn't happen.
You did read the contract before signing, didn't you?
All your rights are not in the contract. The dealership writes contracts to protect them. Check with a lawyer, your banker, or a consumer advocate before making a decision prompted by the dealership. They are not your "buddies". They are in business to make money...period.
Finally a sensible answer. I get sick of hearing its always the consumers fault.
There should be a check valve on the pump to prevent this, it sounds like its defective, unless you have the valve setup that you can fill the FW tank off of the city water hook up, then the valves may be in the wrong postition.