UrbanMan replies: Sorry about the loss of your father but in his death he has probably made you safer with willing you a Bug Out site with running water, albeit a bit of a walk away. Your RV can give you a place to live if you decide to re-furnish the cabin, as the cabin would be handy as you can heat it with wood and you probably can't do that in the RV!
While I have not seen your property nor an map of the area, here are some general recommendations on basic factors for you to consider, and forgive me is I go outside the requested "defenses" request.
On Defenses
I woulds consider these factors:
Observation - the ability to see your threat is a two way street sometimes. Early warning devices such as magnetic, IR beam and camera sensors should be considered but also consider simple methods such as a trip wire which connects a circuit to ring a bell, light a bulb or other visual/audio means if someone drives up your gravel road. Observation also means a vantage point whether it's a ground based LP/OP with a covered and concealed route back to your buildings, and/or, a high point such as the roof of your cabin (after you fix it) or some other type high point. Again, plan for a protected route back to the buildings.
Fields of Fire - It would be nice to have say a 500 yard cleared area around your cabin/trailer so you can see and engage threats before they knock on your door. Map out and make plans for any dead areas that would provide cover for any attackers.
Obstacles - Fences, barriers, barbed wire, etc., are not necessarily used to keep bad guys out, they are used to sort the curious from the evil - the evil will keep on coming. Obstacles and barriers however can serve to channelize the threat into position where it is easier for you to engage them.
Consider anti-vehicle barriers as well, so people can't drive right up to your building or use them as moving cover upon an assault. 5 foot long buried posts with about 2 feet of it sticking out of the ground can be one barrier for vehicle, but consider a defense in depth. That may be followed up with spike strips for tires and as a anti-personal obstacles. Barbed wire can be yet a third barrier, Consider setting up barbed wire right behind the spike strips from your potenntial threat's point of view....they may get tunnel vision on the barbed wire and not be looking for the spikes. Concealment. This also includes disguise. Do something to make it look like your gravel road connecting to the state road looks not to be used. You may want to look at sterilizing any signs of use,...maybe putting up a gate with a rusty locked to look unused.
The below sketch is an example of begijnning a baseline defense in depth, provided you have cleared fields of fire, and have negated any advantage of dead space - that is space that attackers can use to approach and attack that offers cover and/or concealment from your buildings or positions. Also consider blindsides to your buildings from observations. Every position should be mutually supporting from others.
Water Supply
First of all water is a most valuable commodity. It would be nice to have a way, maybe a solar or hand powered pump to pump water from the stream to you buildings/trailer. I woulds consider purchsasing some water storage tanks, 750 gallon to 2,500 gallon, and maybe even several of them to place behind your bldgs/trailer and the access road - so people upon the most likley approach will not necessarily see them. Anybody doing a visual reconnaissance around your property would see them anyway. Having some sort of overhead cover on these water storage tanks to replicate a shed or garage from overhead observation. Even as GoogleEarth updates their archive imagery every couple of years, some bad guys looking for sites to raid or to occupy may see your water storage tanks from the web is they are not covered or disguised.
Food Supply
You will have to grow your own food. This means an appropriate area that you can water and protect; adequate seed supply on hand and the necessary tools. Probably means a green house in the winter time as well as the ability to can your harvests.
Neighbors
If I was you I would find out more about your neighbors with the dairy cows. They may have chickens (think eggs) or other livestock, and be a great source for living off the land such as planting a survival garden. If you can build rapport with them then you may be able to mutual support each other through hard times.
Good luck Jeremy.
