Laws vary from state to state but in generall if a Trailer is over 2,000 pounds GVW it will have a braking system, this includes a break-a-way switch and cable. This system REQUIRES a battery.. So the trailer will have a battery. Also many systems in modern RV's including the lights, and control power for the Fridge & water heater, Furnace, Water pump, Fans, and Control power for the A/Cs, all 12 volt battery/converter power. So there will be a battery.
Make sure you get a good power feed, (If possible come off the main fuse box) make sure you have a good ground as well
Someone mentioned the ground plane for the antenna.. They make NO GROUND PLANE antennas like the one I use. works.. Well.. not the greatest, but still good.
CB in the trailer... When camping. Makes a good "intercom" with the camping partner when same is out and about.. Pick a channel other than 9, 19 or 13, for your "intercom" channel.
CB in the Tow vehicle or Class A. In many areas Truck Stops will get on the air, should there be an "incident" on the highway and advertise their parking lot for use by truckers who do not wish to spend hours in a traffic jam.. Had that happen in 2012 to me.. Pulled off. found parking, enjoied a nice relazing night's sleep. headed on in the AM after the accidents had been cleared. Even if it only happens once every few years it's worth the cost..(And it's not the first time it's happened to me).
HAM radio.. I do not often encounter other hams while RVing, but I've found a few.. including one day at a truck stop.l. one of the Truckers was a ham. Normally I just check into nets at night, but when on the road at least one radio is always on 146.52 MHz FM. Always.. i may start putting on on 52.525 as well but .. Well, that setup is a tad more complex.