Hi Michelle,
There has been a lot of discussion on this subject on another forum,
www.RV.netI know nothing about, nor have any experience with any of the 3-valve Triton engines, other than they were never put into an E-series because they simply don't fit inside the engine compartment because the heads stand too tall.
As I understand it for the earliest 2-valve Triton engines, regardless of engine size and cylinders, the problem is as you say, too few threads in the engine head to hold the spark plugs in. I believe the troubled number of threads is four. It is extremely rare for the plugs to pop out on their own. The problem is revealed most often when the engine is being serviced with a new set of spark plugs, corrosion being the primary culprit. There is some debate when Ford increased the number of spark plug threads in the engine heads from four to I think eight. Either 2003 or 2004 is the start of "good years". 2002 is surely the last year of the four-thread engine heads.
I have been a volunteer mechanic in the C.A.R.S Ministry at our mega-size church for the under-resourced in our community for nearly 20 years now. A lot of early 2-valve Triton engines are now passing through the shop, most often inside old Ford Explorers. Unfortunately it is not uncommon to encounter this problem because these vehicles are old now and have been exposed to Chicago winter salted roads for many years. I could not put a percentage of vehicles with this problem, but it is low. Still a low number is significant.
If I were shopping for a used motor home, I would try to avoid one with a Ford chassis that is not older than a 2004 to assure I don't have the problem. On the other hand, if I found an exceptionally sweet Ford chassis motor home <2004 at a great price, I would not rule out the purchase because of the spark plug issue. Keep in-mind that motor homes are not commuter vehicles that are driven in rush hour traffic, being loaded with salt spray during it's life time. If the motor home never seen salt, I would feel fine buying it.
My brother owns a 1997 Ford chassis E350-V10 Starflyte motor home, the first year the Triton V10 was installed in the E-series. His rig turned 100,000 miles last year so he got a tune-up which went fine, no issues with changing the spark plugs.
As far as which Ford chassis is better? It's very simple, "The Newer, The Better". Like you have mentioned, Ford has been improving the chassis along the way. I hear the latest transmission introduced in 2016 or 2017 is noticeably better than the ~11 year old TorqShift transmission it replaces.
I feel it is best to limit yourself to a Ford chassis not older than a 2008 if buying a motor home in lengths exceeding 28 feet for the sake of weight/load alone. But in the shorter lengths where weight is not an issue, the pre-2008 chassis might actually benefit for it handles the load, yet offers a softer ride on the front axle. Nearly all the extra load capability in the 2008s to 2017 is added into the front axle where it is not needed in a short motor home. My PC-2350 suffers from the front axle being too light, not too heavy, so a softer front suspension seems better to me.
This is only my opinion here, based on what I have read and concluded over the past 10 years on these things. I am no expert on this stuff so please don't hold me to my facts & figures stated here.
Ron