RV water heaters are mounted against an outside wall. Go outside and look for a removable cover that has a mesh vent installed.
Open this cover and the water heater should be inside.

If the water heater is propane your propane tank may be low of fuel. Also very cold weather may not let the propane produce adequate vapor to fuel the water heater. A low level of propane can make this problem worse.

If the water heater is electric, you may have too large a wattage draw from other appliances, when the water heater is trying to heat. Turn off items such as hair blowers, AC, and Microwave when the water heater is on. If a generator is providing the electricity it may not have enough output. If shore power is providing the electricity you need at least 30 AMPs.