The recommended service interval of the 5R110 transmission is 30,000-miles. The only way to get a full fluid exchange is with a heated exchange machine or a lot of inginuity in maintaining an open thermostat inside the transmission.
I recommend a combination treatment initially and then you have a choice afterwards for long-term maintenance:
First the initial treatment:
1. Drain the old transmission fluid. You will only get about 8-quarts although there are techniques that can get more fluid out. Take a sample for analysis at a laboratory. Here are a few that I have used:
Predictive Maintenance Services, Inc.
Oil Analyzers, Inc. - Oil Analysis, Oil Testing Services
There are also over-the-road truck service centers that have a while-you-wait lab.
2. Drop the transmission pan for a thorough cleaning, making sure to give the gasket a good look as well as the magnetic drain plug. The gasket is multi-use so it doesn't necessarily require replacement. Don't worry too much if there is an accumulation of metal on the magnetic plug either. The used oil analysis will provide much more reliable data than the calibrated eyeball. Your first sample has no basis for comparison in any case with a physical observation - take a photograph if you wish to make any comparison later.
3. Replace the pan and refill with Mercon SP or Mercon LV only in the same quantity that you drained. Use the transmission dipstick tube as the fill tube but be careful not to use the oil fill tube - many people will perform multiple fluid exchanges and it is easy to mix this up - be forewarned.
4. Go to your dealer to get a full fluid exchange. Before you deliver your truck for the service make certain they will also replace the external filter. If not, replace the filter before you replace the fluid you drained. I use the NTZ filter from DieselFiltersOnline.com
[NTZ09-C09B] - Ford Torqshift transmission external Transmission Filter(replaces FT145)-Includes O-ring - Diesel Filters,Additives,brake rotors, and brake pads for Chevy, Dodge and Ford Diesel Trucks
NTZ makes the filter for Motorcraft so you will save a little money without sacrificing quality...any other filter than NTZ or MC is asking for trouble. I have purchased BOTH the MC and NTZ filter and have physically seen the NTZ marking on the MC filter out-of-the-box. There are photos online, to include the DFO site.
For long-term service you may either return to the dealer every 30,000 miles or perform a modified user-service such as draining the 8 or so quarts from the pan every year (10,000 to 15,000 miles would be typical) and exchanging the external filter at that time as well. It is a bit more frequent service and will never provide the cleanest fluid but will avoid the extreme accumulation of contaminants either - at least in theory. The only way to assure the quality is to regularly sample the fluid for analysis. Either at every annual "supplemental" exchange or bi-annual (or thereabouts) full heated exchange.
The 5R110 is a great transmission. I have heard anecdotal reports of both techniques returning favorable results. Your choice is to rigorously maintain a consistent maintenance program with objective data such as from a used oil analysis program.
Don't wait for a problem to make a fluid exchange. Get a sample and move forward. You'll be making a wise choice.