You might want to consider buying an Intellisync/Charge Wizard and setting
it up in your garage as a battery charger to maintain your battery(s)
removed from your Airstream.
This charger setup will not overcharge the battery, even if left on all the
time. Overcharging is a real problem with the "Maintenance Free" batteries,
since it is virtually impossible to replace boiled off electrolyte (caused
by over charging)..
Progressive Dynamics sells refurbished/rebuilt converters at discounted
prices, so the cost is modest. Also, a small model (low amp) is more than
adequate for this use. [Or you can buy a bigger one for eventual
installation in your trailer. A 40 amp unit is fine for either use.]
Oliver Filippi