I haven't been in a caravan for years but I seem to remember the spirit level being an essential item. If we didn't get things level it was harder to empty the sink . Has he already got the basic things like a step for getting in and out? A rolling water carrier so he doesn't have to carry it? Being older would he appreciate one of those door curtains that keeps bugs out? If funds allow how about a small porch awning so he has somewhere to put wet coats and boots? Some comfy chairs and maybe a little table for sitting outside?
We have a Motorhome and were given loads of stuff from an emigrating relative. Most useful bits were : Folding Draining rack A draining board and a draining mat to stand wet things on (soaks up the water when no room on rack) Hand-held vacuum cleaner Small Bin Lightweight chopping mats / double up as placemats if needed! Decent looking plastic wine glasses Nice Melamine crockery (but stick to proper "china" mugs!) A roll of non-stick matting to line all the cupboards and shelves with - stops everything falling out.
Best thing ever is the solar panel - but I imagine that will be above budget!
Trying to buy my dad a birthday present and would like to get something useful. He has recently bought a caravan and I would like some advice on a gadget I could get him to go in it .. he is 60 something (just in case that affects what you recommend!!!)