I have done a TON of sound (and heat) insulation in my Axis as well. Its much better than factory but IMO is still a little loud at 75 mph (but not bad at 65 mph).
I dont think (or expect) it will ever be "car" quiet inside.... most of what I hear now is outside wind noise from the mirrors? (and the side windows?). I have changed the angles of the mirrors (folded them in all the way but still able to use them fully) and installed soft gasket material on the leading edge of the mirrors and windows.... but nothing seems to help.
You need to put two+ layers under your hood, bottom dash and on the firewall (all from under the hood). Insulate under the folding desk and everywhere else you can add insulation on the inside.
I also insulated under and inside anything I could elsewhere in the RV.
Its all helps but I think the next big improvment for me will be from installing fender wells with more insuation inbetween them and the body.
But I will still have the wind noise (unless I installed double pane windows).
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2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor
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