I might take a while to dry out the RV ... I wouldn't spend the money on the humidifier ... electric heater and box fan should suffice.
Your rig should have a switch in the bathroom as well as the kitchen area which controls the water pump. RV water pumps are pressure sensitive and when there is a leak or you turn on the faucet they will start up ... if you have a big leak it will run constantly .. if you have a small leak it will occasionally turn on/off.
If you know anybody who has an RV ask them for help .. experienced RVer should locate the problem quickly. Might ask the prior owner.