Most of the small chargers sold to homeowners are like you describe. They have at least two and sometimes three charging levels.
If you were to use a charger in the emergency situation of the onboard converter failing, you'd want one that could charge at a high rate so running your generator would result in a fast charge.
If you were using a charger as a maintenance tool, a trickle charge would be all that's necessary.
I have solar power so it obviates the use of a trickle charger and can keep my banks maintained while in storage. This camping season will also prove if I installed enough panel to supplant the onboard converter. If you have 6V GC2's in the mix, it is good to have a charger that can do an equalize session occasionally. The solar controller I have has that functionality, too.
I do carry a small 6amp/2 amp charger in case everything else fails and also carry jumper cables in the truck. Having a plan "B" and a plan "C" eliminates any worries about the 12V system.