As K1VSK said, it makes no sense at all to reverse the EchoCharge device. Among other things, you'd limit your charge on the house batteries to no more than 15 amps (the capacity of the EchoCharge), even if you had upwards of 100 amps available for the house batteries. Dumb, for sure!
Cam said it well, but it bears repeating: starting batteries require very little charging. They only draw a miniscule amount of power when starting the engine (lots of amps but only for a few seconds). Typically, this power is replaced in just a few minutes after you start the engine. After that, what's the alternator to do? Idle?
EchoCharge devices will typically begin sampling and charging the start battery when the voltage exceeds about 12.8. It doesn't have to be 13.0 or more. Furthermore, the charging voltage on your house batteries will reach that threshold very soon, even if they're badly depleted. Remember...it's not the resting voltage of the batteries which causes the EchoCharge to activate; it's the charging voltage. That will reach the necessary 12.8V in just a few minutes after the engine starts and the alternator begins charging the house bank.
Finally, the 50mil draw on the house batteries is for all intents and purposes negligible in a properly designed system. As Cam correctly pointed out, if you anticipate being off shore power for long periods of time, you must provide some other charging source (solar panels, wind generator, etc.) to keep your batteries topped up, otherwise you're gonna kill your batteries through sulfation, stratification, and the other ills which attack chronically undercharged batteries.