Any lead acid battery powered system, such as a seldom used starter battery or RV, boat, and travel trailer house batteries, will undergo self-discharge over time. Lead acid batteries that discharge below 50% of their fully charged capacity will have a shortened life expectancy. If they sit dead for very long, they may stay dead and will need replacing. To avoid this wasteful expense, some RVs come with small maintenance or trickle chargers, such as 1 to 5 watt solar panels. The salesman may have said it will keep your batteries charged, implying you just use all the power you want and it will magically come back. If at the end of one day's moderate use, you park, lock up, and don't use the RV for a month, the trickle solar charger may recharge the batteries in a few weeks. The real purpose of solar trickle chargers, however, is not to recharge batteries, but to maintain the batteries by charging at a rate equalling or slightly exceeding the rate of self-discharge.
Properly sized solar trickle chargers are cheap and simple to install. No charge controller is needed, just mount or put the panel somewhere and clip it to the battery. The maximum size of the panel is determined by the size of the battery or battery bank. A rough rule is that if the panel puts out about 2 watts or less for each 50 battery amp-hours, then you don't need a solar charge controller. Starter batteries are not rated in amp-hours, so use a small 1 to 3 watt panel. If you have one battery and it's a common Group 27 with a 105 amp-hour rating, then a 4 watt panel is the biggest to use but a 2 watt panel would work. If you have a 210 amp-hour battery bank, then the solar panel could be up to 8 watts but that is the maximum size, not what is absolutely needed. A common trickle charger size is 5 watts, and one 5 watt solar panel would be enough. If you have an 840 amp-hour battery bank, then 5 watts might be enough to equal self-discharge, or might not as rate of self-discharge varies from 3% (maintanence free AGW batteries) to 20% (some cheap batteries) per month depending on the battery type and manufacture quality. So it is best to err on the side closer to the maximum. Going beyond the maximum without a solar charge controller can result in overcharging which damages batteries.
Smaller panels can be plugged into the cigarette lighter and put on the dash. Mounting horizontally on the roof is one option and mounting vertically on the side works too. Screws are an option, but running a bead of Goop glue, or PL Premium glue, held in place till dry with duck tape, also works. A box cutter blade could remove them if that ever became necessary.
The panels don't have to be in full sun. Full sun maximizes their power output, approaching the rated value, say 5 watts. If a 5 watt panel only puts out 2 or 3 watts, however, that may be enough. If the panel never sees direct sunshine, but sees only the sky, even if cloudy, then a 5 watt panel may still be putting out 2-3 watts, and to counter self-discharge that may be enough. Where you live also matters as if you live in Chicago a panel will put out about half what the same panel in Tucson puts out over the course of an average day. If you live in Chicago, think of going with the maximum 2 watts per 50 amp-hours of battery guideline or use a larger panel with a solar charge controller.
Consider the battery size or size of the battery bank in amp-hours you want to maintain while sitting unused. If you neglect to keep the batteries charged and they self-discharge, the next time you go to use the system or start the motor, you may end up buying new batteries costing an order of magnitude more than a solar charger that would have saved them, and you the trouble replacing them. A solar charger: $25. An RV that starts: priceless.
So if you need a solar trickle charger, consider:
1. Starter battery for motorcycles, small personal water crafts, ATVs, snowmobiles and tractors; clip on battery type: Sunforce 50013 1-Watt Motorcycle and Powersports Solar Battery Charger
2. Starter battery for car, RV, boat; clip on or plug into cigarette lighter and put on dash or elsewhere: Schumacher SP-200 2.4W Solar Battery Maintainer
3. Large battery or small battery bank, 125 amp-hour or larger: Sunforce 50022 5-Watt Solar Battery Trickle Charger
4. 15 watt panel for larger battery banks, comes with 7 amp solar charge controller so could be used with any 12 volt battery: Sunforce 50033 15-Watt Solar Charging Kit
If starter battery and house battery or battery bank are separate, use two trickle charging panels (Sunforce 52022 5 Watt Solar Trickle Charger - Pack of 2 ).
To use solar to charge battery banks, to live off-grid without a gasoline powered generator or just not have to run a generator, then an array of solar panels, a solar charge controller to avoid overcharging the batteries, and an inverter to run AC devices is needed. Such systems are not simple, not easy DIY ways to go solar. But going solar is doable for the price of an effort and some money: start with a 100 watt system and expand up to 400 watts if need be: Solar Panel Starter Kit 100W Monocrystalline:100W Solar Panel. If you need to mount to a curved surface, flexible panels are available (Renogy® 100W Monocrystalline Bendable Solar Panel ).