A pair of 140 watt solar panels will solve your problem.
One problem that you might not be thinking of is that the RV uses 35 AH daily to just run the CO meter, propane detector and refrigerator. This load is about what one 120 watt panel will produce in 24 hours. By installing a pair of 140 watt panels, you will make more power than you consume most of the time, and if you ever did need to run the generator, running it for a few minutes in the morning will provide a great start to a full charge, and the solar can finish charging over the next 7 or 8 hours.
I have a 400 watt solar system and 4 batteries. With more panels, you do not need to save as much for overnight, as you will be recharging each morning pretty early. This saves weight too!
SunElec.com had some 140 watt panels on sale for $229 rated at 12 volt nominal output and have frames to mount them to the roof mounts.
I made my roof mounts from 6" long aluminum angle 2" wide. Three holes 3/16" into the roof with #10 screws and plenty of rubber roof sealant, and a 5/16" hole for a 1/4-20 bolt into the panel frame on each corner. This way you will also be able to circulate air under the panels, cooler produces more amperage.
A PWM controller is only about $20 for a 20 amp version. The MPPT controller does not provide enough 'extra' power to justify it's cost anymore.
Yes you can plug in the external charger, but it will also make the built in charger think the battery is 'full' and at 14 volts, so it will stop the bulk charging sooner, and prevent the 100 amp charger from working at full capacity, so it might actually charge slower than without the external charger. You would need a DC clam on amp meter to know if that is correct or not. .
Better to go with solar, and then you will have full batteries at the start of each campout! And the solar will make up for most of the loads used during the day too, with the battery only being drawn down a little bit at night....
Charging a battery has been described well by comparing it to a frog jumping towards the end of a log. The first couple of leaps are pretty big, but as it gets closer to the end of the log, each jump is very small, until it finally takes the last couple of steps to get to the end.
Same thing with the battery bank. If it is at 11 volts, then your 100 amp charger can put out 90 amps for a short time, and then slowly will slow charging as internal resistance and built up voltage on the plates reaches 13 volts. Amperage will slow as it gets closer to 70% full, and really slow down in the 70-80 % full mark. That last 15% can take all night to get in 40 amp hours, while in the first hour, you can usually get 40 - 60 amps per hour when the battery is really low.
Good luck,
Fred.
