chassis year 2003; rv/coach 2006

Call a Ford dealer you trust and ask how much to change the plugs. Plugs are different today than the vehicles we grew up with. Make sure you tell them 2003. If they know their stuff they will know the difference between 2 piece plugs (2004 and up) and what you have. Might have to give VIN number. If you get dealer involed, also install new plug boots and cop springs. Fuel injectors need replaced when they go bad, not at certain mileage factor. If you do it yourself, don't be surprized if you find a little rust or water in plug holes, very common. Just the way water drains from wiper area. Not cracked head gasket or anything falling apart. No big deal and very dependable motor. Also get coolant changed, like the plugs, coolant is not like old days. Needs certain type, not just pick something off shelf by price, requires stuff that cools and keeps rusting down to min. And that is reason to change coolant, fighting rust not coolant ability but fight rust fighting.
I enjoy learning to do maint items on ours. I get a warm fuzzy feeling when I get a dealer bid of $1,300 and did it myself and ends up costing me $400.00 for parts. I only use Motorcraft OEM replacement parts, except on very few items. And I'm assuming maint hasn't been done for awhile. Maybe other owners did change out some things. ALL motors have their problems or known weakness.