Solar Panel is the best option for charging the batteries.
I have been traveling the West for 30 years now. I rarly stay at an RV park. In some states like New Mexico and Utah there are many secluded areas you can camp for free. Every year I stay around the four corners longer and longer. Last year I stayed out West for 7 months. I find that power and water are two most important in the wilderness. There are few things you can do to extend the battery life like converting all the light to LED. With regular lights the battery may last 4 or 5 hours at night. But LED it will last for weeks. The biggest use for water is dishes, so I use paper plates, disposable bowls and cups. Over the years I have tried using generator but generator is fine for short term use but is a poor option in keeping the batteries charged. Anyone who has tried to charge a deep cycle marine batteries by running the generator for an hour knows it does not do a good job. Marine batteries charge best by trickle charging at low amps, typically 2 to 4 amps all day.
So in recent years with the cost of the solar panels coming down dramatically I have installed two 260 watts solar panels on top of my Fun Finder. It was the best investment I ever made. I experimented trying many options of using the panels and the most logical one I decided is mounted flat fixed on top. To carry a large panel capable of charging enough power and constantly loading and unloading was not feasible. But mounting two large ones flat on top makes much more sense. It does not maximize on the solar, but tring to unload or keep adjusting it on the roof was not a good option. I expected 50 percent efficiancy. Which is still pleny of power.
Here is the reasoning. It is always charging. While I am driving down the highway, parked , anytime it is charging. Solar regulator keeps charging when needed and shuts off when not needed. When I am ready to use power there is no work to be done. I can get on average 200 watts or more in the middle of the day. When the sun comes up in the morning, the power gauge shows 2 amps around 7am and steadily climbs to 10 amps at 10am to 20 amps or more at noon. At typical marine battery does best charging at 2 to 4 amp. I have 3 hooked up for the road. I do have a nice Yamaha generator for AC when needed. But with solar power I run all my equipment and a swamp cooler out west and never need to run my generator. As the sun goes down and at night I have plenty of power because the 3 batteries are fully charged from solar. What's amazing is that on the cloudiest day I am still getting 3 to 6 amps of power.
From my great result, I think everyone need a solar panel. My 520 watts system, I spent less than $1000 for everything. If anyone is interested in how it is installed, I will post photos. I am an engineer but installation is very easy...
As to the question if 45 watts is enough to charge your battery. In general you need to consider that a 100 watt panel will delivery about 60 watts in real life. So 45 watts will deliver about 27 watts, little over 2 amps. If you want to just charge one battery all day without use, then it's perfect. But if you are planning to use other things like fan, computer, etc, then you need to get a larger panel.
