Asda-Walmart at Cribbs Causeway (just off J17 M5, and only a couple of miles from the M4) has a huge car park, and often has a number of trucks parked. The access is also reasonable for large vehicles, and the petrol (diesel) station also has HGV lanes. Of course, check with the staff before you park up…
Don’t try to go there on a Saturday, though (especially the ones in the run-up to Christmas).