Ford publishes a maintenance schedule (including one for heavier duty use, dirt roads, towing, etc.). So that's a good base.
You may already know/do this, but I find it is useful to arrive with a pre-printed list for them to use. Then you keep a copy and at the end you go through it with them. (I also of course tell them I'm open to doing other things if they see them, and then sometimes get a second opinion if those things come up.)
Here is a 1998 guide I have saved. You can probably find a 2003 as well, although I have a friend with a 2003 and not too much has changed in terms of overall maintenance.
Without even looking at that though, I can tell you some of the things I have done or looked at (this is my second similar vehicle, previous one having been an E-250 camper van).
First off, unless I have impeccable service records from the previous owner, I expect to spend "extra" getting things up to a baseline that I KNOW myself. If any of these are known (i.e. actual service records, not vague ideas), then I might skip them. My camper van came to me at 15 years old and 95,000 miles; the Chinook at 15 years old and 49,000 miles.
1) Change all fluids (coolant/brake/differential/transmission/engine oil). On the differential, make sure they know it is Limited Slip (if it is, I believe it would be). The Ford dealer I went to didn't have a clue. "Oh then never put that in motor homes." "Really, then it's funny it's on the spec sheet and in the VIN..." Reason to know is they use a different fluid than a non-Limited Slip axle.
2) Change serpentine belt (keep removed one for emergency spare).
3) Change spark plugs* (*make sure you are confident in the person who does them) Even though they say to do them at 100,000 miles, I did them on my Chinook when I got it due to age (also did them on the van when I got it but that had 100k).
4) Air filter, oil filter.
5) Inspect brakes, renew as necessary, lube slide calipers in any case (they can stick/smoke/make you have to pull over/be expensive). Since you will have rear disc brakes, have them lube or whatever they do to the parking brake adjuster (drum in hat).
6) Check date code on all tires (you can do this - there are many guides online that will be easier than me explaining it in words, but basically it's on one side of each tire, in a week/year format). If over six years old, get new ones even if "the tread is perfect." The rubber ages out and dually problems on the road are no fun. Along the same lines, if you have the rubber valve extenders, consider true extended stems (Tireman, Bog - I think I have a post explaining and showing these).
7) I think I had wheel bearings done, but I can't remember the interval, so perhaps check the recommendation.
8) Thermostat (new).
9) Check upper and lower radiator hoses.
10) If you can park over concrete (or cardboard), make sure there are no leaks or drips that you'd want to have checked/repaired.
I'm sure I've forgotten some things, and of course we're not even getting into coach/generator at this time.
I will say that I "split" the work when I got the Chinook. I was in a large city far, far from home, and didn't know any independent mechanics (and it was 20º and I needed to get going!). I researched and went to the best rated Ford dealer in the area and had them do some work. Not only was it expensive, but they messed up most of it and I had to make a fuss and get some corrected by them and some (at my hassle and expense) later. Boo! I'm sure there are great ones, but even then you never know who's going to work on it after they wheel the gurney away.
So a month or so later I went to an independent shop and had them do the bulk of the work. Less expensive, better job, and I could talk to the mechanic face to face and they showed me things while it was hoisted up (I also was able to stay in the parking lot overnight, plugged in and all). Just something to consider.
I have not been to one, but I guess they also have Ford "fleet" type mechanics who specialize in the heavier vehicles (E-350/450/550 etc.). Not that there is a whole lot on an E-350 that is bigger than a regular van, but maybe they are more attentive. I have read of some Chinookers having found some real "gold" in this type of Ford shop.
Also consider going to specialists: Tire shop for tires, etc.