Depends on how many humans & dogs in the TT as well as how often the shower is used and how much cooking is done. I doubt many would cook or shower without the fans on, but exterior (make-up) air needs to be drawn back in and if vents are closed tight, it can reduce the amount of exhausted air. An adult human body releases around 0.35 lbs/hour of moisture (approx. 0.35 pints) from respiration and transpiration (which adds up in a 24 hour period) and could help in sizing a dehumidifier.
If a longer TT, because there is no air circulation fan in a TT, perhaps a smaller unit at the front and another at the rear would work better? You may want to look at the dehumidifier's capacity at it's rated temp. and relative humidity level vs. the temp. and RH you want/will have inside the TT as lower values will reduce performance. A single larger unit can't hurt and may have some benefits as noted above.
You still should have the vent(s) open a little to vent stale air due to CO2, maybe formaldehyde gas, odors, etc. Having a window and vent cracked open as far apart as possible helps to get cross-ventilation.
I would definitely use a hygrometer to monitor the relative humidity to try and maintain it below 50%.
