While there's no direct plug-and-play replacement for the Atwood GC10A-4E with a gas-only tankless unit, you can upgrade to dual-fuel tankless heaters like the Girard GSWH-2 or Truma AquaGo. These maintain electric capability while providing instant hot water and better efficiency.
AquaHot cycling every 15 minutes at idle is not normal. Possible causes include heat loss through stuck zone valves, thermostat issues, or circulation pump running when it shouldnt. Check that all zone valves are closed and pumps are off when no heat is called for.
Spring clamps can be reused if in good condition. Worm gear clamps are easier for future service but must be quality stainless all-band type. At 138K miles, hose replacement is wise - check all hoses for softness or swelling.
Your 2023 Holiday Rambler Nautica's persistent problems with the Aqua Hot system, leveling jacks, and multiple warranty issues indicate serious manufacturing defects. You need to escalate beyond your local dealer to Holiday Rambler corporate and consider lemon law protection.
Aqua Hot engine hose failure requires coolant hose repair or temporary bypass to Aqua Hot. Carry spare heater hose, clamps, and coolant for roadside emergencies. Bypass Aqua Hot if needed to get to shop.
The 2008 Monaco Diplomat is generally a solid Class A choice, but focus your inspection on the Caterpillar engine maintenance records, slide-out operation, and electrical systems. Verify all major maintenance has proper documentation and consider a professional pre-purchase inspection.