I used 2 inches of the pink sheet foam in the walls and roof and I can't believe how warm the trailer is. In fact, on our last trip, it got a bit too warm and we had to figure out how to vent the trailer without opening the top vent in the cold rain. Thank goodness I put windows in on both sides. I feel confident about staying warm in any type of weather. I do have a small catalytic heater inside to warm things up but I've never run it for more than 20 minutes before shutting it down and staying warm for the rest of the night.
It sounds like you may be a bit too far along now to change wall thickeness but if you can, I'd recommend putting in the foam sheets. I think I paid 17.00 for a 2" thick 4x8 sheet
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"Indecision may or may not be my problem" Jimmy Buffet
The Road Foamie Build Thread: viewtopic.php?t=45698