I installed this unit. Paid about $700 for it. It is the closest I have found to your size requirement. It keeps the water hot for 24 hours.
BX Series Water Heaters
Fast, Efficient Heating
By:
Quick Windlass
Sku# 185822
Item ID QCK BXS2512SL
Mfg# BXS2512SL
SPECIFICATION
Type Square
Watts 1200W
Exchanger Yes
Weight 29.8 lbs.
Dimensions 13.0"H x 13.0"W x 18.9"L
Gal. 6.5 Gal.
Collapse
OVERVIEW
These are high quality units incorporating an extra long, large surface area, SS engine heat exchanger and electric incoloy steel heating element with an adjustable thermostat. They incorporate a pressure relief/non-return valve which also acts as a drain for seasonal decommissioning.
• Orientation: horizontal design, but works well in any orientation
• 316 stainless steel tank, argon welded for corrosion resistance
• 304 stainless steel case
• Heat exchanger is a thick walled (1/16"), coiled 10 ft. tube of 316 stainless steel, so it won't compromise the engine cooling system
• Insulation is 2" of high density polyurethane foam
• SS mounting brackets included
• All units are 1200 watts; 110 VAC (220 VAC available by special order)