RVs often have a converter. It takes 120 volts AC and makes 12 volts DC to run lights, pumps, etc. The control circuit for a propane fridge would also be a typical load.
People often confuse converters with battery chargers. Typical converters are not good battery chargers as they either don't charge enough or charge too much. Either way the battery suffers. Converters don't need a battery to work. They power the lights, pumps, etc.
Battery chargers aren't great converters. They need a battery. Some get confused by a changing load current and decide to shut down.
Trickle chargers are supposed to keep a battery charged making up for internal self discharge in the battery. Usually they are cheap, poorly regulated, and ineffective. Some do their job well enough. That job doesn't include pumps and fridges.
If your RV stays plugged in you don't need to keep a battery. A power supply or converter will work with no battery. If you occasionally go camping off grid you might do well with a $100 solar panel, $100 flooded lead acid battery, and a $15 PWM charge controller. Check / add water monthly. If you can't, won't, or don't add water get a $200 AGM battery. Be warned, AGMs can be fragile, easy to ruin overcharging or deliberately undercharging to avoid overcharging.
The Harbor Freight solar panel is neither monocrystaline nor polycrystaline. It is amorphous making it much larger for the same watts. I have seen complaints that the glass isn't tough. It's good enough for stationary use, maybe a problem at vehicle speeds.
