why does it only hold a charge for a few days? are there loads connected to it? typically there are some 'parasitic loads that will run down a battery in a short time. have you installed a battery disconnect switch to completely remove all loads from the battery? a fully charged battery without loads will keep its charge for several months. have the batteries been discharged to the point of damage? if so, a trickle charger may not keep them charged? pull them out and have them tested.
2015 cardinal model 3825fl
2015 dodge ram 3500 dually