I put a 10 watt solar on my batteries to keep them charged but 10 watt didn't do much but I have a dual battery system. For long term storage I'm sure 18 watt would keep 1 battery charged but I'm sure you can get a lot bigger solar kit for $100 bucks. I'd sho p on the net. Try eBay. A bigger solar is always better and the charge controller will keep the battery from overcharging the battery. I just tried the 10 watt cause I had it in my barn already. A 100 watt will give you about 60AH of charge per day with good sun just to give you an idea of charge capacity, I hope that's right. I have 190AHs of battery in my Trailer. So you can see it would take weeks and weeks to charge your battery with an 18 watt solar cell. 30 or 50 watt would be a lot better.
2014 Palomino Columbus 3650TH
435AH 12V VMax Charge Tanks, 520 watt Monocrystilline Solar Array, Morningstar MPPT TS-60 CC,Morningstar Remote,Cotek True Sine Wave Inverter,Cotek Remote Controller,50amp Progressive Hardwire Surge Protector
2015 Dodge 3500 6.7L 4X4
2011 CanAm Commander XT 1000