I put one layer of Dynomat stuff (50 mill thick asphalt base material with thick alum. foil) then one layer of Fat mat (3/4 inch foam with thick alumium foil) under the dog house. (lots of diff name brands on Amazon).
And then 1 Dynomat and TWO Fat mat under the hood/firewall areas and anywhere else I could stick it too. So every square inch is covered when you look under the hood and then some.
It made a signifcant diff in sound and heat (no heat transfer now) inside the RV.
A dash mat might Help a little but I think you need to start under the hood area and dog house first.
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, Fresh water surge tank, Black Tank Tornado flush, 4999 zip ties.