Most inverters would have to be mounted in a water/dust proof environment. With adequate ventilation away from fumes from the batteries. Any that you buy will come with instructions for wire gage and circuit protection requirements.
After you've got all that done you would need to pull an " A " pillar moulding off next to the windshield to see if there is room to run a power cord up. If there isn't a route for the wire apparent. Then your going to need help from some one with the exact same class C, that you have, and has done the same thing, to figure it out.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.