We periodically plug in to 110v, 15A in order to top up the house batteries and also the night before a trip, so that the rather ancient dual voltage fridge can get nice and cold on 110V before we drive off and put it on 12V. Never had any problems. I think you got the key element - as long as you do not draw at or near 15A, the SMB doesn't know the difference. I, too, laid out a lot of $$ for new AGM house batteries and I want them to outlive me!
1993 E350 SMB, 15M, 84k miles - a cheap date
2006 Toyota Prius because we like clean air
2002 Honda Odyssey - can haul lots of stuff
1972 Mercedes 350SL with 4.5l V-8, just because
Wag more, Bark less, Play well with others.