At this point, the only downside to LiFePO4 batteries are that you shouldn't charge them directly from your alternator. You need a DC-DC charger - the voltage of the two systems are slightly different, and you don't want your alternator trying to shove 14v into a fully charged lithium battery.
If you haven't stayed up to date with batteries, you'd be shocked at how much the industry has changed in just a few years. And prices have fallen like a stone. If you learned about "lithium" batteries 3 years ago, it's time to re-educate yourself.
For example you can get a 50ah LiFePO4 for about $150, which is less less than the price of a 100ah sealed lead acid deep cycle battery. They have the same usable capacity, but the lithium is less than half the size and weight. The lithium battery will last longer.
I'm getting ready to buy a 8kwh (650 amp hour @ 12v) battery for $900. It's a single box that weighs 150 pounds, and would replace a 10-12 lead acid batteries.
Pros for LiFePO4
- Light weight (a truck battery size weighs about 25 pounds)
- More capacity for the size
- Use of all the capacity, unlike lead (you actually get 100ah from a 100ah battery)
- Longer lifespan than lead-acid
- You can get fancy battery management systems that give you all the data
- You can get more capacity in a single battery, saving $$ on cables
Negatives
- Alternator charging should be done with a DC-DC converter
- Charging below freezing will reduce the lifespan - but
- some are so small it would be easy to keep them inside the trailer
- some have low temperature charging protection and won't take a charge when it's too cold
- some have built in heaters
