White smoke coming from the exhaust is normally water leaking into one or more combustion chambers. The water vapor should disappear soon after leaving the exhaust pipe. Most such leaks can usually be repaired, but sometime cannot.
A qualified professional needs to pressure test the cooling system in order to determine where the leak is actually located.
You state the mechanic said the leak is from a "bolt into the head". Does this mean an actual head bolt or maybe a manifold bolt into the head?
A heli-coil is not difficult to install, but is rather labor-intensive.
At this point, any repair done by a good professional will be rather expensive. Amateur attempts will only add to the problems.
Unless he is giving the coach to you for free, my suggestion is to pass on this rig. With the interior issues as-well-as the engine, You are probably going to end up spending a lot of money. There is always something better out there.
I don't mean to sound "preachy" here but I was a mechanic for more than 45 years and have an old Winnebago with $9k in it and it will never be worth more than $1500.00.