Hi Paul
Your best to check and flush the water pipes out plus also check you battery-this needs to be kept trickle charged when the van isnt in use. Check taps for cracks if water has been left in them over a cold spell. Pop the heater on for an hour or two so it airs the van out. Check tyres for correct pressure / damage such as cracking. Make sure your water tank is ok and again no frost damage has taken place. Look for damp - check the rear corners and front corners on your Elddis - it is after all an Avante but as a Barrons dealer special. Check gas is coming through ok and also pipe and regulator on bottles are all in good order too.
If all ok your ready for the off-the Firestorm range was very popular and sells well on the used market, I can see yours is the 505 a good used layout too where the rear can be left up as a bed if required so you have the advanatge of a fixed bed van yet it can be quickly taken down to get your living space all back.
Have a great touring season
Andy Jenkinson