We built a simple battery charging system to keep our home backup generator battery happy.
Have set up systems for a couple of our camping vans and a outdoor shed. Systems are able to keep out fishing batteries charged or other devices.
Latest system uses a Renology 100W panel from Amazon, PWM solar charge controller and double pole breaker mounted on a DIN mount from Aliexpress. Our charge controllers also have a couple
UBS outputs to run lights or charge things.
Do it yourself shouldn't cost more then $175.00
Same type of system could easily keep a tractor, ATV or boat happy.