I'll first start out by saying I'm not a big fan of the current Optima batteries since Johnson Controls took them over a few years back. Their longevity and CCA output have suffered.
OK, you say that you hear 'clicking' when the start button is pushed. That means one of several things, with the easiest to check are all connections, especially ground. Is your ground close to that battery and have you checked it for rust, paint under that bolt or for looseness?
Next, that battery - here is a good primer on how to do checks: Battery Service - Autoshop 101(you may need to copy and paste).
Then, that clicking solenoid or relay - are you sure that it's actually engaging properly. That is, the core or moving contact(s) actually connecting with the stationary contact(s). If using a jumper and it starts while directly connected to that relay or even the existing battery, it may provide enough extra amperage to pull that set of contacts together. If this is the case, then really question the battery cranking ampacity, cable size (they need to be at least 4 awg with heavier better) and the connections.
Always remember when chasing problems that RV manufacturers don't always use the best materials, do much in the way of quality control or employ the most stable work force to assemble them and screwups happen