If you have owned the van from new, then you have a 3 year warranty on the heating system, just as you have on all other appliances fitted during build of van.
Your dealer is obliged to fix it for you.
However, the tank should only be just above minimum when COLD, and just up to max when HOT
Your assertion " Typical Bailey 1Yr warranty on this appliance" is wrong and misleading.
Extract from Bailey Warranty on Touring Caravans
3 Year Manufacturer’s Warranty
For a period of twenty-four (24) months from the initial date of purchase Bailey of Bristol offer a comprehensive warranty on all parts and components as well as full coverage for any manufacturing faults forming part of the original specification of the vehicle, with the following specified exceptions:
The following items are covered for one (1) year from the date of first registration Microwave Ovens Pioneer Stereo Radio/CD/MP3 Players & Speakers
The manufacturer’s warranty then extends to an additional twelve (12) months on the following items.
Chassis: all chassis members including corner steadies Suspension: axle suspension and braking system (excluding any damage to or faults in brake drums and shoes that are caused through misuse of the braking system or from normal wear and tear) Running Gear: road wheels (excluding tyres) Towing Mechanism: all mechanical components fitted to vehicle (excluding electrics) Cooker: the cooker unit including burners, grill, oven, flame failure device and igniter
Refrigerator: door seal condenser, gas control valve, gas igniter, flame failure device,12 and 230v heater elements, gas thermostat, 230v thermostat and 230v temperature control switch Water System: water heater (gas or electric), fresh water tank, water pump, water gauges, taps and shower head Electrical System: mains hook up input connector, ELCB, battery charger and distributor unit and interior lighting units (excluding bulbs) Cassette Toilet: the cassette toilet is covered (excluding seals, valves and glands) Heating System: thermostat, motor, switches, control unit, gas heater, flame failure device and igniter (excluding ducting and fittings) Windows: the functionality of the opening and closing system (stays, handles and catches) and a warranty against the cracking of the acrylic Upholstery: zips, seams and colour fastness