Due to the high current an inverter can draw they are usually located close to the batteries, that's probably why it is behind the rear wheels. Obviously sealing the compartment will eliminate the water and road grime, but as you identified could cause the inverter/converter to overheat. Rather than opening the breaker, is it possible to provide a different opening in the compartment where water/dirt will not enter when driving? On my rig, the openings for ventilation in that compartment are on the inside (directed towards the differential) and I never get any moisture in the compartment. Good luck.
Lou
05 Travel Supreme Envoy