You need a cheap digital hygrostat for monitoring your indoor relative humidity (RH) and to compare to the one that may integral on the dehumidifier you need. This website allows you to get a working sense of the relationship between air temperature, RH, and mold risk.
Dew Point Calculator
Play with the slide bars and watch the mold risk indicator change. Ideally you want to maintain any trailer to less than 60% RH most of the time. Having a higher indoor RH can cause problems if it happens too many hours of the day or too many days of the week. Lots of rain, tracking water indoors on your feet and clothes,
leaks, etc. will increase your indoor RH to higher levels, but typically they will drop down again in a few hours. You never want to allow your stuff to get moldy or mildewed; the health risks are very consequential and there is no way to decontaminate moldy objects that have any porosity to them, like clothing or wood.
Ventilating to stop mold in a trailer is very risky. It has to stay warm enough inside to warm up and dry out the outside air that is coming in, and then that warm wet air has to leave the trailer without condensing on any trailer parts on its way out. You might get away with this in east San Diego or Phoenix...
A any compressor dehumidifier is also a 2 COP heater. (2 COP means half as much electricity for the same amount of heat production as any electric resistance heater or
light bulb (COP 1 or less)). It is cheaper and safer to run a dehumidifier than a heater. Note that most portable AC units can also be used to dehumidify, and these days you can also buy portable heat pump units that can cool in the summer, heat in the winter, and dehumidify whenever needed. A dehumidifier also very efficiently heats the space as it removes water from the air. I have been heating my house in western Oregon partly with dehumidifiers for 20 years. A dehumidifier or a portable heat pump would be the cheapest way to heat a place with electricity with a portable appliance. You have to carefully read the installation and use instructions for portable AC/Heat Pump units to be sure they are doing what you need them to. A 8,000 BTU portable AC unit is about the right size for plugging into the wall, or a 20-30 pint dehumidifier.
If you get a dehumidifier with a hygrostat integrated into the control, it will turn itself off when the RH setting level is reached and come on again when RH goes up. This is great except the hygrostat in the dehumidifier is often not very accurate, so you need to compare your little digital hygrometer to see if the target RH levels are being maintained and tweak the settings as necessary. Also, hygrostats typically have a large hysteresis, so you may have to set the control to less than 60% RH to be sure the space never gets above 65%. I set mine to 55% to be safe. I took a Honeywell hygrostat designed to use in a house and put it in an outlet box with an extension cord for a more accurate control when the one on the dehumidifier got too far off.
Many dehumidifiers/portable AC come with a garden hose connection so you can drain the bucket continuously into the kitchen sink or bathtub (trailer drain valves locked open!), and you definitely want this feature so you can leave the trailer unattended for months as needed.
Note also that there are two different temperature ranges of dehumidifiers available. The ones you use in the summer in the midwest only function at indoor air temperatures above 64 degrees. The ones you use for trailers in the cold season need to operate all the way down to 42-45 degrees. The cold temperature ones are not hard to find, you just have to be sure they are rated for cold weather operation.
This Ebay seller beats the Amazon price for the previously-linked Davis heating appliance by almost $30, as usual.
https://www.ebay.com/itm/Davis-Instr...oAAOSwcBRbvAsz The problem with a low capacity system like this is that if it gets overwhelmed by unplanned events your trailer can get moldy and there is no way to clean it up.
I don't know about you, but I never have any unplanned events in my life...