Often, on cars which don't come with HIDs as standard, the problem is just crap wiring. The wire is just too thin and/or long once it's been round a loom. The right way to remedy it is to wire in relays by the headlamps, activate them with the original headlamp feed and run a full-fat +12v straight to/from the battery. My Citroen had this problem and the difference with relays is like, well, night and day.
^ this. OP doesn't say what sort of camper it is, but if it's a VW T3 or T4 then the headlights are notoriously poor on account of the wires going all the way from the battery across the van to the headlight switch and fusebox and then back to the lights so you only get 10.5V across the actual bulbs. You can get relay kits for about £30 that just plug in.