If they replaced a slide, you could be smelling an adhesive or cleaner.
A battery that is gassing excessively will smell like rotten eggs / sulfur.
A refrigerator that is leaking will smell like ammonia.
If you have ever used starting fluid, you will know what ether smells like.
The "misconnected house and engine batteries hookup" could be the cause, as a battery that is being overcharged or experiencing a heavy load will outgas excessively. I would return to the shop and have them verify their work.