At our park we had a water riser go bad under a trailer. It was about six feet under the trailer. We used our little Kubota bx backhoe to dig as far and deep under the trailer as we could. We then hand dug under the trailer with a shovel and a cheap electric jack hammer with a spade bit from Amazon ($300ish total). I dug the hole extra deep with the backhoe so my helper could hand shovel the extra dirt spoils where I could remove them with the backhoe.
One day to dig. One week for the plumber to show up and then another day to fill in the dirt. Took about a week using water till the dirt finally completely settled.
If you end up trying to dig it out in house it will be well worth the money to buy the jackhammer. Shoveling dirt is relatively easy… Breaking it up is hard. I just looked up jackhammer on Amazon. Cheap Chinese jackhammer with spade bit $229.99 (Oct 2019) with shipping. I have had mine for about ten years now and it is still going strong. the only things I’ve done is add oil and motor brushes. Guy in pic is helper.