I have a 2005 Ford F-150 with 8,000 miles, its used mostly for trailer towing and a little bit around town driving.
How often should I change the engine oil/filter and transmission fluid.
I currently change the engine oil/filter every 6 months, still on the original transmission fluid. The truck as the towing option included (IE a transmission and oil cooler)
6 month interval for oil is great.
Transmission fluid you may want to change within the year. However that could likely go years without changing and ill effect with your low mileage.
6 months oil/filter is fine.
trans fluid I would check once a year and change every 5 years if it stays clean-looking.
I would also get an automatic battery charger ~5 amps and charge the battery every month or two, especially in winter.
If you are towing, change your transmission fluid every 20-25k miles. How heavy is the trailer?? If your truck doesnt presently have one, an external trans cooler is a must for vehicles that tow, for extra cooling. Heat will damage an automatic transmission. Towing causes excess heat. An external trans cooler is cheap, easy to install and is great insurance for your trans.
transman
Why don’t you want to follow the instructions in your owner’s manual?
“If you are towing, change your transmission fluid every 20-25k miles.”
That would take him 10-12 years. Just sayin’.
Service intervals in owners manuals are a crock… Remember that owners manuals are written by the vehicle manufacturer, the same ones who advertise low maintenance costs, lifetime transmission fluid, 7k, 10k oil changes etc… Those are great selling points, that would make any unknowing person want to buy that vehicle. Then, after you buy and you go to your local dealer for service they push the 3k oil changes, transmission flushes, brake flushes, injector cleaning, Power steering flushes etc…
transman
GOOD POINT… He puts fewer miles on his rig than I do
I only know my relatives ignore the maintenance requirements and run a slightly used vehicle 175k-200k miles/8-9 yrs with simple 5k oil changes, proper timing belt and repair as things(not much) happen. I believe they change out the air filters themselves. Most of the fluids they believe are lifetime even though stated to change out 
It has worked with a Subaru Forester, Toyota RAV4, Dodge Caravan, VW Jetta, Ford Van/pickups without breakdowns.
The cars are well used up in the end but they simply move on. They spend far less on repairs than the maintenance per book. I think cars are well designed to take this lax maintenance for 150k at least hence the recommendations for new cars.
Of course as always, transman is right. To prove his point, you only need look at an 07 Uplander or 09 G8 owners manual. The Uplander indicates a 100k interval, the G8 claims no fluid change is needed, ever.