Electrical problems.

Hi.  Completely new to caravanning and have just completed our first week away in our newly acquired Rapido Club 1994. We had the caravan serviced before we left and the engineer sold us a used leisure battery as there was none in the van when we bought it. When we arrived at the campsite,we  found that the thetford toilet does not flush. Everything else worked on mains hook up except the 13 amp plugs fitted inside. After 4 days on site the electrical power dimmed slowly then went out completely leaving us without any power.  Now everything else is dead.  I have checked the power cable and the consumer box and all the switched are on.  All this  is OK. Plugged everything in at home but still nothing. Any ideas as to where to check next would be appreciated.