I have to disagree, four or five spells per scene is a ton.  Plus, I've always felt like Harry just has multiple sources of "sponsored" magic to draw upon and permit more spell casting than your typical wizard.  Mantle of the winter knight, demonsreach, soulfire, hellfire, etc.  Not to mention the variety of enchanted items he uses.  Just incur some debt to eat the stress and permit more casting.  Harry also typically carries a gun or uses mundane attacks (fists and such) in combat.  He isn't throwing spells all the time.  Casting and spending that associated mental stress is something wizards should have to manage.  The sponsored magic option provides a way to cast more frequently, but at a price.

Between enchanted items to create spell effects and sponsored magic, I've never felt a need to house rule further free casting or extra mental stress.

General concept is a travelling wizard who wanders across the country.  No family.  Circus is a good example.  Similarly, I could definitely see a Romani caravan as warranting a threshold due to their community, traditions, beliefs, and multiple generations.  This mobile home is more like a mobile bachelor pad - almost equatable to a "homeless" person living out of a cargo van.

Looking back over the rulebook:

Most thresholds or other things that ground out magical energy (like a source of running water) have a base strength of Fair (+2). ...basically, every mitigating or contributing circumstance should add or subtract 2 from this base total. In the case of thresholds, the circumstances you’re looking for are things that make them “more of ” or “less of ” what they are.

Seems that this particular case, the mobile home in question would be a step less than a standard home, so a +0 or +1 at best.