I don't think you can use the trolling motor with 4AWG wire. You would have to splice to the 10AWG wire on the plug, which would mean you really should fuse the circuit for 10AWG, as the rating is determined by the smallest wire in the circuit. If you upgrade the wire, you will also need to switch the connector for a higher current plug that can accept the correct wire. That said, using the stock wire (which is rated to 60A) that you already have and using a DC-DC charger may be a more cost/effort effective solution than upgrading the wiring.
As to airbags, I have given up on getting mine leak free. Sometimes they will hold pressure for months, sometimes they leak out in a day or two. My solution is an on board compressor with a paddle valve. When on a trip I use them to level, so we check/adjust the pressure every day.