Looking for a small DC DC charger

I'm putting a 7AH lead acid battery in my camper shell, I need a small DC DC charger to keep it topped off. Any recommendations?

I'm considering using a small solar charge controller from the alternator. Any reason I shouldn't?

 

Why not just get a small DC DC charger

https://www.renogy.com/12v-dc-to-dc...ZM0nqiFfkMYVxBKFT8N_HaOCWlZmpGjoaAmuMEALw_wcB

 

There, that's what I'm looking for most of what I could find were much larger and well into the couple to several hundreds of dollars.

 

Solar controllers are typically looking for a good bit more than 12-14v from an alternator, so a true DC DC is the way to go

 

That's what I understood, but someone made that recommendation and it didn't sound legit. Closer to 24v then either pwm's or uses magic for MPPT to charge the battery.