5 watt... that's nothing.
I think you're wanting to use the load ouput to connect the panel? That probably won't work since I don't expect the load output allows a reverse flow (charging into the battery).
If you want to use the 5W panel to drive the MPPT.. no way. First I don't think the voltage is high enough, second, the mppt probably won't do anything with soo low power. Add in all losses it will probably use more to keep the controller on, than the input of the panel.
A small solar panel can work to tricklecharge, but connect it directly to the battery (with an appropriate fuse off course). And imho 5W is useless, since all panel ratings are based on maximum solar radiance. I would go for 20W or so if you really need to tricklecharge the battery (And want to be able to charge at least SOMEthing, that 5W panel probably works only during full summer sun)