The lithium phoosphate batteries weigh half as much as the lead acid ones so weight it not a concern. Best if you add additional batteries close to the existing one to minimize current loss from a long cable run. With a slide-in camper I owned there was only space of a single Group 31 battery and so I welded together a cage to hold a second battery under the floor and with the AGM batteries the second one could be on its side. Today I would replace the AGM with a lithium as step one.
An AGM or flooded battery is best discharged only 50% so one gets 50 AH out of a standard battery. With a lithium phosphate battery the discharge can be far great and at 20% SOC it can provide 80 AH or 60% more charge for RV use. What counts then is how much charge can be replenished during the day.
The other good thing about the lithium phosphate battery is that it can be recharged at a much higher rate to take advantage of available daylight with solar panels and to also reduce charging time with a generator.
I would measure the space where the single battery is located and see whether a larger battery can be put in its place. With my camper there was enough space to replace a Group 24 battery with a group 31 size battery. Along those lines, the Lithionics battery is Group 31 size and provides 125 AH or 25% more than with a Battle Born or similar lithium battery. More expensive but if space is in short supply they are worth the premium.
https://lithionicsbattery.com/produc...h-g31-battery/
One big advantage of a trailer is that usually there is a lot more real estate on the roof for adding solar panels, including the larger size very high output panels. With our motorhome I switched the flooded batteries for Lithionics one and I added an additional 290 Watts of solar panels to the roof (as much as there was room available).