From my research, the LI-BIM225 would be the easier install. You could replace the existing relay under the hood with the BIM and be good to go. A DC2DC charger will be a little more challenging to wire in although I believe it will be easy enough and plan to install one soon, just that I haven't got around to doing so.
The reason I chose the DC2DC charger (Renogy 40a) is that it is a constant charge rate and will prevent the alternator from running at full output and will charge the (a) LifePo4(s) batteries completely. What I don't like about the BIM is how it operates and in particular these two statements.
"Under normal charging conditions, the Li-BIM225 will connect for 15 minutes every 35 minutes. That means that the BIM will connect for 15 minutes, disconnect for 20 minutes, and repeat this cycle until the coach battery is charged."
"The Li-BIM does not
guarantee 100% battery charge, but prevents harmful battery charge levels"
and the datasheet download.
What I would expect to be happening behind the scenes with a BIM is 100% full charge occurring (with a low LifePo4 battery) and the alt at full output for 15 minutes, then no charge, then 100% again, then no charge. I would prefer a lower rate of charge that is occurring anytime the engine is running. ~CA