I'm thinking mobile home meaning limited space so maximize the effects on walls, ceilings and smells.
YYY to fairy lights.
YYY to pine cones and I have an aromatherapy oils in Christmas mix (possibly Neal's Yard) but you could make up one with some pine, mandarin and cinnamon type oils.
Hang cards either on strings/ribbons/strings of beads along the walls or blue tack to the doors.
Hang garlands from ceiling and stick things to the wall like snowflakes or garlands or Santas/Snowmen etc. With a 5yo, make some newpaper angels or snowmen for windows. Tinsel along the bottom of kitchen cabinets (as long as not dangling in the way of kitchen activity and not near the cooker).
If you have a safe lantern, make use of nightlights, or get battery-powered versions.
Depending on where you have space for a tree, get a small fake for a tabletop, a narrow one to stand on the floor, a flat-backed one for against a wall, or a tiny real one (even some off-cuts branches tied together might work really well in a pot). There are lots of options, just think about the space you have and what might work.
Winter in a mobile home may be chilly, so bright Christmassy coloured throws to use as well as decorate could be helpful and practical as well as decorative. Cosy fleeces (Ikea and Primark both do nice but cheap fleeces - and you may get lucky on nice colours) would be great as easy to wash and dry too.
You may not want to get Christmas "ware" for space/storage reasons, but you could get party versions to throw away - tablecloth, paper plates, plastic glasses etc - even if its just a pack of small bowls to put out the little bits of treats for the season. Or 1 nice - maybe wintery rather than Christmassy - proper mug each for your coffee.
And possibly think about outside the mobile - is it on a site that you could hang a wreath on the door or near the door on a pole or similar? Could you put a tree outside and decorate with outdoor decorations (well tied on plastic items, your glittery pine cones, a string of tinsel well tied on against wind/rain, a set of lights (solar?) suitable for outdoors)? Even a "Santa please stop here" sign.