First things first, what all are you planning to power? Often the cheaper inverters are very optimistic about their wattage rating. Grab a kill-a-watt meter and do some testing.
Secondly, you would probably need to install a high idle circuit in the vehicle. Extended idling is bad for engines. Commercial (f-250 and up) trucks are going to be wired for this, at least last time I checked they were. If I recall my 7.3 diesel had a wire for 1200 rpm and another for 1300 or 1400rpm, again it's been a while.
Lastly, I'm no electrical engineer (my dad is though). Putting two AC sources together would require that they be in perfect phase or they will drift, the voltage will drop and both of the units would fry trying to fight each other. So, not a good idea.