RV Repair: electrical short in 1999 ford tioga, dual batteries, electrian

Question
I purchased a 1999 27 ft ford tioga this year. When I purchased it everything worked. Then when I tried to use the shore line cable to plug it into the 30 amp outlet in my garage. It shorted out the main electrical line to my house. I had an electrian come out and repair the electrial line it was a $800.00 bill. Then he came out and tried to plug the shore line back into the 30 amp outlet and said that the motorhome has a short in the electrical system. Could it be the shore line cord needs to be replaced? or is it somthing else? I also always have to charge the dual batteries on this motorhome. Please let me know.
Thank you for your time.

Answer
i would follow the shore cord into the unit inspect the cord and the junction box.
it could be a receipt inside the unit or something inside grounding out you would have to shut all the breakers in the unit off and test the system and turn one breaker on at time
it could be the wires going from the juction box to the converter
becareful or take it to a shop that can test this for you