We've got quite a few things underway with sorting out the motorhome. One fairly easy option was to switch all the lights to use LED's, as those use a lot more power than the halogen and flourescent lights that our motorhome has as standard. There were quite a few different types of lights and each needed a different approach:
G4: The reading lights above the beds use little G4 bulbs and those can just be replaced directly with LED versions. For example where a 10W halogen G4 was used this can be replaced with a 1.2W LED, which gives a similar amount of light.
GR8: The main lights in the motorhome's living area and in the bathroom were fitted with flourescent lights with bulbs with a GR8 fitting. While you can get direct LED replacements for those it is a little confusing as it depends on what type of starter is used. In the end what I did was to remove the flourescent bulbs and disconnect the 12V supply from the transformer that feeds them (I think the bulbs themselves might be 240V ones) and instead use it as the source for 2 of the G4 bulbs, using bases that can be had for pennies on Amazon. That seems to work pretty well and it'll be easy to reinstate the original lights if necessary.
T5/G5: The lights above the dining table and above the kitchen area used 530mm flourescents with a T5 fitting. Again direct LED replacements could be had for those, but would also require the starters to be bypassed. Instead I bought a couple of 12v LED light bars from Amazon for £13.99 for the pair. I then removed the complete light fitting and wired those in place (a 5 minute job). The strips each have 72 LED's and consume 4.5W instead of the originals 13W - and are also brighter.
So for a cost of about £25 in total all the lights have been replaced with much more efficient versions, which should be a help when we're wild camping or using Aire's without mains electricity.