#1: You don't want to be without converter. Upgrade it, so that it wouldn't boil batteries and would output 13-14V without battery.
#2: Converter already IS a multistage charger. They are 3-stage. Illiterate Amazon descriptions might talk about 4-stage though.
#3: Solar won't have any adverse effect on other charging sources, install it if you want to minimize generator time and charge battery fuller.
#4: You don't want to ditch the alternator. There are ways to isolate the charging of starter and coach battery, if you need to.
