A solar charger would be ideal for the trickle charge. Small panels run at 1 to 2 w but it may be a good idea to get a slightly bigger unit and purchase a Charge regulator/splitter to allow two bikes to be connected at once and the charge maintained in both. The larger the rated wattage the better the unit will work on maintaining the battery in bad weather. I am based in the UK so have no idea where you would get them in the US. I could put up links to UK suppliers so you can see what I mean.
One point. Most of these things are advertised as being suitable for use outdoors but on the cheeper panels the fittings etc are definately not good quality and rust very fast ( particularly in my installation where salt water is involved).
I got round this by mounting the panel on an IP65 box and running the wire into the box. Seal the wire where it connects to the panel with silicon and make all the connections in the box and ran two wires out with stainless croc clips.
Also note that solar panels and long term snow don't go together well