Trickle battery charger

Have a look on eBay there are numerous options 

You have posted under the 'Storage' section, is your motorhome actually in storage or at home? Difference being solar or mains powered charging maybe!

CTEK are a well known and respected brand.

I use Optimate chargers, I have two, both low amp versions mainly for my motorcycles, they take a few days to bring a leisure battery up after use, but the chargers are ideal for keeping the leisure batteries charged and maintained when MH is not being used.

I've got a CTEK MXS_10. Got it principally for my car battery (stop/start AGM battery) but it's a brilliant piece of kit. Will charge battery banks up to 200Ah and maintain banks up to 300Ah. Has a Comfort Connector you can permanently install on the vehicle so you only have to plug the charger into that or you can also use croc clips, it's also IP65 so splash and dust proof. Not cheap but a really good piece of kit.

Another vote for CTek. The 5 amp one should be adequate. I have two as we used to leave two cars on charge when going to Europe with the motorhome.

We bought the smart charger from Alidi with has a trickle charge mode

These look interesting, 

https://cpc.farnell.com/kemo-electronic/m083/module-battery-charge-reg-12v/dp/HK01163?mckv=s35Um21XH_dc|pcrid|224646539664|kword||match||plid||slid||product|HK01163|pgrid|49734053271|ptaid|pla-817183934360|&CMP=KNC-GUK-CPC-SHOPPING&gclid=CjwK