Can you post a picture of the water heater rating plate?
If it was a house inspector used for house sales, he has no authority and probably no knowledge of circuit breaker other than about 80% of all homes do have 30 amp breakers for water heaters. But many mobile homes original water heaters were 3500 or 3800 watt element and your wiring may only be 12 gauge. Some only had one element. Mobile homes are built for low cost and if it passes code, 12 gauge will always be installed rather than 10 gauge. If it is a typical mobile home water heater it is probably 30 gallon under a cabinet or in a small closet space. The plumbing connection is usually throught the side of the tank, not the top. Water heaters placed in regular homes built in the last 35-40 years are usually 4500 watt elements but 3800 watt units are still available for older homes.
25 amp is not usually used but you need to know the gauge of the wire to the water heater. It could be 12 gauge which requires 20 amp breaker and the water heater might have been replaced with a 4500 element heater, therefore the 20 amp breaker would be prone to tripping to someone put in 25 amp breaker. This all on assumption.
Your breaker panel may not be manufactured anymore and maybe why 30 amps are not available. If this is really an issue for closing have a local licensed electrician that is familiar with mobile home work to sign off on what is there that it is perfectly safe, if not he can correct it and have the receipt for closing.