Yes, the tender will charge more because of the power usage from the unit. Why not then just use the 'shore power" (which he is doing anyway with the tender but may be leaving the batts hooked to the unit) and keep the batts topped off with the convertor? No need for a tender then as the converter will power up and down as needed. Am I missing something?
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler