Is the truck wiring 12 or 24 volts? If it's 12, the truck will regulate the voltage as it does for it's own battery, so I'm guessing it's 24 since your asking. I would just buy a solar charger and leave it plugged in all the time. I bought one for an unregistered shop truck at work. It didn't get used much and was only driven on the property, the battery would never stay charged. I mounted it on the dash and wired it in through a battery switch so it would never be back fed when the truck was running. Worked great and the battery was never dead again, the truck would fire up instantly after sitting for weeks.
I'm sure you can mount the charger out of the way on the front of the trailer or even route the wire and put it on the roof where it will be hidden. There are cheaper options, but I used a battery tender brand from amazon.
http://www.amazon.com/Battery-Tender.../dp/B004Q83TGO
Bill