I've never undertaken such a large project, but I would assume a key driver of your costs will be whether the pipe runs under the mobile homes, or behind the homes in what is hopefully a wide 'breezeway.' If the pipes are under the homes, then the new pipes will need to be hand-dug under the homes. That's hugely expensive, but still cheaper than moving a home out to the middle of the road, digging with a backhoe, and then moving the home back into place. On the other hand, this may be a perfect time to explore using the new pipe bursting technology to replace the orangeburg. On the other, other hand, perhaps you could simply trench new sewer lines in the breezeway, if any, and abandon the old pipes.
Interested to hear what your situation is and what bids you receive from contractors. That'll be a key piece of diligence to help you decide if you want to buy this park.
My 2 cents worth,