If dumping your water will put you to the right weight then yes, as for fine a good number of years ago i got put on a weigh bridge with my sprinter van, and although I was 300 kg under max I was 60 kg over on my front axle, I got fined £60
It was a similar incident although i didn`t get fined that led me to uprate my GVW from 3,850kg to 4,100kg through SVTech many years back.
I popped into the free one just outside Stirling to check which i normally do if i`m passing, VOSA were in there doing stop checks.
One of their guys came over and asked if i was ok and i said i`d just popped into to do a quick free check but seeing as they were working i wouldn`t interfere.
He said no problem, they were having a break and did i need any help, i said i`d done it many times so he stepped back and let me get on with it.
Lined myself up, pressed the button, waited for the sequence to start then drove forwards slowly, the wife had pen and paper ready.
Front axle, rear axle then gross weights all came up, the wife had written them down so i pulled out of their way so i could look at the results.
The VOSA guy came over and clearly saw the frown on my face and asked what was wrong, i explained the GVW weight was fine but the rear was slightly over.
Turns out he was nearly a motorhomer himself and planning on buying one next year when he retires.
He asked where the water tank was situated and when i told him he said instantly drop the water and you`ll be fine which i did.
Due to mine being a low profile with rear garage it is physically impossible to get any of the weight further forward than it is now and the increase in GVW all went on the rear axle which is exactly where i needed it so it was money well spent and kept me legal.
I bumped into him several years later on Sango Sands campsite, he recognised me by sight and my registration number.
I explained what i`d done in upgrading the GVW and showed him the new VIN sticker to which he replied it`s a shame more don`t take the weight of the motorhomes more seriously and they just chuck stuff in regardless.