water tank plug left open for two years??
Better flush bugs/insects out good before reinstalling the plug. If thats the case you may want to disinfect your fresh water system.
Check DOT date code on tires especially after sitting with weight for two years. Replace if 3-5 years old and know problematic ST C/D/E tire brands. If the trailer will be making long all day runs I would replace them with a good tire for trailer use made to go the distance (mileage and years).
Like grit says run every thing now.