I thought the Fox sisters later admitted they created the knocking noises by popping their toes?
One of them did - and the body wasn't found until 1904 - it was in a wall, so most likely they did not know about it. The person who the entity claimed to be was never even proven to have existed.
The 'rappings' also followed the sisters wherever they went, which didn't really help them much either...as far as believability is concerned.
One of the sisters showed how it was done for money (both of them were alcoholics) and later recanted her 'confession' - probably because she needed more money and wouldn't be able to make any as a medium as she had been doing.
I'm really sure this is just the case of an over-active imagination of a lonely child being validated by the coincidence of finding human bones underneath the trailer. I wouldn't be surprised if a large portion of trailers have some sort of bones in their insulation - human or otherwise - due to animals making homes and hiding things in them.
Depending on where it was in Georgia, the bones could be from any time period - even colonial times. The trailer could have been parked in an area that used to be a farm - over unmarked old family graves.