Few thoughts:
It takes 4200 joules to heat a litre of water by 1 dec C. So to heat 5,000 litres by 50 deg, you need to put in 1.05 billion joules.
1kWh is 3.6 million joules.
So, assuming no heat loss in your static you'll need 291kWh, which will cost around £35 a night, just under £9,000 a year. If you heat over 12 hours, you'll need 8 x 3kWh immersion heaters so you'll be drawing about 24kW, around 100A. You might well need professional help to solve that.
If you did it with gas it'd cost roughly £12 in gas a night.
However, the problem with gas is that the capacity of a normal central heating unit may well not be enough so you'll need an industrial heating expert. Then again, you'd take your annual heating cost down to £3,000 so that'd be cheap at the price.
Incidentally, you'd also produce around 25 tonnes of CO2 every year, so you might want to consider offsetting that in some way.
Vin