Welcome - it's difficult to answer your questions because there are so many variables involved.
How many panels can you put on your roof? That would depend on the panels you choose and how you lay them out. Flexible or hard panels can be used but if you plan on leaving the OEM panel installed you may want to get more of the same panels. You don't have to have exactly the same panels everywhere on your roof but it helps if they are similar in style and output.
Since you already have one solar panel then you should also already have a Solar Charge Controller on your travel trailer. You'll need to understand what brand and model solar controller you have installed and what it's total current limitation is. Many Winnebago motorhomes come with Zamp 30-amp solar charge controllers that limit the total wattage on the roof ~500w. I don't know what Winnebago installs on their travel trailers. So, that's some info you'd need to learn before moving forward to add more panels.
If you put a LiFePO4 battery on the tongue of your travel trailer you will need to address the CONVERTER (not Inverter) because it's the converter that is charging your battery when on shore power. And, it's not very likely that the OEM Converter is lithium ready. Lithium batteries have a different charging profile than Lead Acid batteries and your OEM converter probably isn't capable of supplying the proper charging.
You should be able to find the model number of your converter on the inside of the door covering the fuse/circuit breaker panel. Then you could look up that model and see if it's got a Lithium charge profile or if it can be modified to provide that kind of profile.
The internet is full of RV Solar info and the more research you do the better you'll be prepared for this somewhat complex topic. Also, you'll find a good many of the RV solar suppliers and installers are based in Arizona. So, you may want to work with one of them if you are not planning on doing everything yourself.
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad