303 does a reasonable job on tonneaus, though it is better suited to harder plastics, in my opinion. Leave the armour all products alone, they're going to make a greasy mud on the tonneau at the first sign of dust.
The most important thing is to give it a good scrub first - use some APC and a leather brush, or similar, to shift dirt and stains. If you're using 303, try spraying it on a microfibre applicator and really working it in, rather than just wiping it on. Let it dwell a minute or two and then wipe over thoroughly with a MF towel to remove any excess - shouldn't have too much trouble with it running then.
PM sent as well.