I had a nice post going but then the forum zonked out on me. I'll try to be short this time.
As for your questions on the disconnect, I would say you would want to disconnect the batteries or install a disconnect switch if your don't have one to avoid the parasitic draws in the trailer. I know nothing about controllers.
First, I'm not a solar expert but decided to do some of the math for my situation recently and this might help you decide if that 1.8W is enough for you.
Start by reviewing this article
or Google others like it. Its basic but it gives a start.
Here's my math. I have two Trojan batteries with 260Ah. I live in Wisconsin and its winter so assume I should lose about 8% of my battery capacity each month or 0.69Ah per day (260 * 8% / 30 days).
I can expect between 3.5 - 4.5 of sun hours per day. You can look up solar maps for your area and the month here
If I divide the total capacity loss per day (.69) by the total number of full-sun hours per day (4) I get 0.17A or 170 mA as the desired minimum current of the panel.
I don't know this, but I am sure the manufacturers are optimistic with their figures so I would plan to double that to 0.35A needed from the panel. It seems that a charger up to 5W would be sufficient for me.