IMO return the 7 amper if they will let you. $140 for a 7 amp charger is crazy IMO. And $400 for a 25 amper? Holy Cow! A proper 55 amp converter is well under $300 (ordered from the States--stay clear of our RV dealer stores :( )
What to do? Keep the existing converter for when on shore power. Get a real charger (45-60 amper) for fast charging when you need to do that, and also for its higher voltage to keep your batteries healthy by gassing them instead of only seeing 13.6v all the time.
You can make your own portable fast charger by just getting a deck mount converter and use a cut-off end of heavy gauge jumper cables for the wire/clamp set. Too easy.
BTW, this one here for example, has a goofy 3 stage charging profile but you can take the lid off and twiddle the little blue voltage pot inside and make it into a single voltage charger at your chosen voltage--like 14.8v
.
It then will do two-stage charging (bulk and absorption) at your chosen Vabs till the batts are charged right up, then you can turn it off and go to your regular converter to do the Float.
http://www.boatandrvaccessories.com/powermax-pm3-55-55-amp-power-converter.htmlHere is the same thing but with cables--but you can make your own cable/clamp set by cutting the end off a set of jumper cables
http://www.boatandrvaccessories.com/powermax-pmbc-55-charger.htmlIf you want the best version of this it costs more, but has great features--adjustable voltage by external knob so no need to take the lid off and twiddle the little pot inside and maybe fry something by accident, and a voltmeter on top option. You can still make your own cable set.
http://www.boatandrvaccessories.com/powermax-pmbc-55adj-converter.html