For your highway driving anything above 70 mph is going to hurt. The rpms you mentioned sound good for cruising, the engine isn't working to hard and you are close to the power band.

I would definately pick up a spare that has some life to it, you never know what typoe of condition your going to be in when/if you need the spare. A bald spare is worthless in the snow/ice/rain. A sem-used 235 should be dirt cheap.

If you are getting cold while sleeping in the back of your truck, pick up one of those inflateable air mattresses(and a 12v pump). The air mattress will give you added insulation against the cold from underneath. The flanel or fleece sleeping bag inserts are nice to have since they store easier than blankets.