Sorry, but we don't have enough information to give the advice you want. Here's my take on some questions to start with.
Indeed, you can "over panel" a solar controller as you suggest, but you need to know what you are doing re the specs and panel voltages and such.
We need to figure out what in your system is behind the 500W limit indicated by the manual. Is it the controller itself or wiring between the panels and the controller?
Does the manual indicate series or parallel panels? Typically it would be parallel with a PWM, but need to know for sure. If the manual does not provide enough detail, we need to know the PWM controller specs.
Do you have enough information on the existing panel to know what panels you can get to expand without tossing the factory 100W panel (though more panels like it from the RV maker could work, that could be very costly, more costly than starting over).
Without more information, I would go with Joes suggestion of getting an MPPT controller and also suggest 600W or 800W (three or four 200W panels for lower per Watt cost). You would select the MPPT controller to handle the panels you go with (MPPT controller output current rating and input voltage limit are the critical parameters and need to line up with the panel arrangement ...... likely three 200W in series for 600W or a series/parallel setup for 800W.
There are companies like Rich Solar that can set you up with the right combination of panels and controllers and tell you whether your existing wiring is adequate.