Interesting battery analysis

After contacting the WFCO tech services, I learned that to install a Lithium ion battery I must replace the existing converter, which consists of the 120 volt AC circuit breakers and the 12 volt DC fuses.  He further indicated that the term Control Charger and Converter were really the same thing.  He indicated that I must replace the existing model WF-8735P with model WF-8735-PB-LIS.  The newer model provides the option to switch from Lead-Acid to Lithium to AGM.  The only problem is that the sources of supply are out of stock, so my project may be put on hold until the parts become available. 

Re positive vs. negative grounding, that refers to which polarity in the system wiring is connected to the trailer and tow vehicle chassis. That is the negative buss in all but some now antique cars. So yes you need a solar charge controller that can be negative grounded, meaning the negative conductors in the circuit are connected together and never opened, so that the charge controllers battery voltage regulation action is done on the positive side of the circuit.

There is sometimes confusion about positive vs negative grounding effects on the solar modules themselves. Some solar modules operating at high voltages (as in a big residential or commercial grid tie system) need to be positive grounded to protect them from potential induced degradatiin (PID) which occurs via leakage of ions through the glass and plastic encapsulation of the solar cells. While PID can be a big deal with some solar module types operating at high voltages under some conditions it's really a non issue for our little 12V power systems.

Re connecting a solar module directly to a battery +1 on not doing that. First and foremost, there has to be an appropriate fuse or circuit breaker protecting the wiring from the battery to the solar module. That fuse should be placed in the positive conductor (to maintaing negative chassis grounding as discussed above) as close as possible to the battery because the battery is effectively an unlimited source of current if you short it out. The solar module can only produce as much current as it has sunlight shining on it so it doesn't need a fuse (unless you have more than 2 parallel connected solar modules which we don't usually need to get into discussing here).

The other reason not to direct connect is that you can overcharge your battery which especially in the case of Li batteries can cause a really nasty thermal runaway effect resulting in a fire.