So I am putting the tripping of the garage breaker down to there being not enough amps/voltage available?
I would not lightly dismiss the breaker tripping.
As plcpitt posted, your garage may be a GFI circuit, and either of two things could've tripped it:
(a) momentary surge of current as the inverter powered up
(b) the Ground Fault detector saw a problem...
Let's talk about (b): there used to be an RV electrician who posted here who was dedicated to educating owners of the potentially fatal dangers (and frequency) of ground faults in RVs... both from mis-wired campground outlets *and* faults in the RV's wiring.
If the vehicle frame ends up (even lightly) connected to the 120v feed, and *you* establish the connection between it and "earth" by grabbing the door as you're stepping up from the wet grass... you're toasted.
In that webpage (
http://www.rv-dreams.com/rv-electrical.html ) i provided earlier, they discuss simple test devices. You might buy the best of them.
If you have a voltmeter (or run out and buy one, or even get the "free" ones offered by Harbor Freight), check that (when it's plugged into shore power) your Sprinter's metal does *not* have a potential (voltage reading) relative to a good earthed electrical ground.
(you can probably use the 3rd pin of a standard house-socket as a first reference point)
If you see more than (perhaps) 1 volt, GET IT CHECKED. (it should be zero, but long extension cord runs can bring to view minor ground-discrepancies in your house wiring)
If you have a gadget like a "kill-o-watt" power-consumption tester, you can measure how much current/power you are using *right now* in your "plugged in and charging" configuration. This will be a good number to know (hardware stores or local power companies frequently have loaner kill-o-watts, but they're only $30 or so).
If you find that you're only drawing (say) 5 amps or so, yet you still trip a 15 amp GFI breaker, then *CHECK IT OUT*.
Kill-o-watt/kill-a-watt:
http://www.p3international.com/products/p4400.html
http://www.amazon.com/s/?ie=UTF8&ke...vptwo=&hvqmt=b&hvdev=c&ref=pd_sl_8xd9yt0bzq_b
--dick
p.s. my home-built "system" just uses a typical 15 amp plug. But i do carry along a plug-in GFI unit to place between my Sprinter and the campground outlet. If it trips i dig out my voltmeter and check the power kiosk's outlets. I have encountered kiosks with swapped power and neutral wiring. (the wider blade of a 15 amp socket should be at (very) near-ground potential.)