Here is the downfall of buying a "gutted , or partially gutted" ready for your dream renovation trailer.
The chances of someone having your tank that wants to sell it is pretty minimal to none. I would look at trying to get images of the OEM tank and getting a plastic place to build one for you or find one with dimensions you can use in the spot you want to use it in.
Next time, buy a trailer with everything there, because replacing the stuff you haven't got is not going to be cheap, if you want it stock; especially that panel; BUT lucky for you there are new products out there that will do the same thing and better, ie smaller multi displays that will monitor many different aspects of your trailer on one display, opening up more storage for you and getting rid of the big panel.
Getting a new converter is easy, pump is easy, battery box is easy, furnace, you have many different ways to go that are better than a furnace; Platinum Cats for one, in floor radiant heat grids, Precision Temp radiator fan heating.