With AGM you don't need to be concerned with hydrogen outgassing during normal operation, but there is still the danger of thermal runaway and catastrophic failure. I think you're prudent in planning on a metal box- just be doubly sure to make sure that all terminals are insulated in case the batteries move during transit. It might be good to line the sides of the box with plastic to reduce the risk.
Mounted low just in front of the axle is the perfect place for ballast (batteries).
Have you considered the charging side of the equation? 440 amp hours of AGM can handle a HUGE charge current. Probably the best value/output for a combined inverter/charger/transfer relay system is the Xantrex DR line. The size would depend on your chosen generator but the DR2412 will charge the batteries at up to 120 amps and put out 2400 watts of MSW AC. The charge portion is temperature compensated with a remote battery temperature sensor (so no thermal runaway risk even at high charge rates and a bad battery).
Price is about $880 vs. $2000+ for the true sine wave units of similar capacity. The DR1512 offers 70 amps of charging for $680. I think I'd pay the extra amount for the 2412 assuming you're running at least a 2000 watt generator. It will allow you to run AC & microwave simultaneously.
Store offering them: