It all depends on how much you want to spend, I have one of the cheap $20 1.5 amp harbor freight battery tenders on my 30KW diesel home standby generator it has been hooked up for 6 or 7 years and is still working. I change the battery out every 5 years regardless if it needs it, just because I don't want to ever have to deal with a dead battery during a storm. By contrast I have an BatteryMinder model 2012 which sells for $90 on amazon on my old diesel 7.3 F250 pickup (2000 model) which only gets driven occasionally to haul stuff on the weekends, again it works, gives a few more bells and whistles than the $20 Harbor Freight model, I don't know if it really worth an extra $70 though. (I bought it on one of their lightning deal specials a year or two ago for about half off during the holiday season.
(my motorhome has 400 watts worth of solar panels on the roof which keep the batteries charged)