In an nutshell, yes you can cheat and do that. Because it is short term and you don't plan on withdrawing or adding large amounts of amperage. However what CT Wanderer posted has a lot of merit. Pianotuna may chime in and post his balanced batteries wiring diagram.
But in order to get your money's worth out of the modification it is best to make certain all terminals and battery posts gleam when making the connections.
CHEATER'S TIP
Use jumper cables
Connect the positive jumper cable from the first battery positive to the last battery positive.
Connect the negative jumper cable from the first battery negative to the last battery positive.
It may not end up looking pretty, but I imagine your goal is to do the best you can with what you've got. No moving of the original battery connection wires. Easy and effective. This will do it, even if you should have a high draw inverter and use it.
Have a great vacation and give a smug little smile at the extra battery capacity.