Open the main saloon floor hatch a crack and hold a lighted cigarette, cigar, stick match, lighter or ??? next to it and watch the smoke or flame to get an idea of just how restricted fresh air flow is to your engine room. Take a look at the openings under the vent cowlings to see how small or large they are.
On Dreamer, a 45' Alaskan, I installed a large engine room vent on each side deck. It made a big difference.
Air change is the only way to keep the E R as cool as possible without A/C, imho.
But I installed the added vents to provide sufficient fresh air to the diesels. They loved it!