You should always do a compression test whenever a mechanical problem is suspected to be sure the engines health is in top shape. The load on the engine with low compression is gonna be hard to start. On the trailer there isn't any load on the impeller, so it may start fairly easy, but not in water. Here is some information to doing a compression test;
You will need a good brand compression tester. Go to Auto Zone. I think they sell for like $25.00. The compression gage, will screw in the cylinder head in place of the spark plugs. To test compression, remove both spark plugs. Place spark plug caps on the plug cap studs near the cylinder head to ground the empty caps from causeing a fire from arcing. Using the correct adapter for the threaded end of the tester,(same length of the spark plug threads length) screw in the tester in one plug hole. Hold the throttle wide open. Push the start button. Watch the compression gauge, when it peaks out at the most compression, let go the start button. Read the psi number. I would do it 3 times to be sure it is accurate. Check both cylinders the same way. The ideal compression is 150 psi per cylinder. If it is less, it's not a problem as long as they are close to being the same. If the psi is less than 120 it might (IS) time for a top end tear down and a rebuild. If the psi in 1 cylinder is say 140 psi and the other is 80psi you definately need to tear down and repair. This difference is a lot then there is the problem. I hope this helps you.
There is one thing between the Carb's fuel pump and the fuel chamber inside the carbureator. It is the needle valve assembly. Gas either gets past it, or it does not get past it. It will gravitize to lower engine case, take up the space and have
compression Under the piston in that cylinder the liquid has routed to.
How can gas get past a needle valve other than what we have talked about? One way is if the fuel tank pressure release check valve in the OUT vent line is not functioning. The fuel pressure in the tank overcomes the spring tension on the needle valve and gas is FORCED past it.
If the ignition timing were off enough, it could be hard to start
If the gap of the RV is too loose it could be hard to start. Same if it has an air leak, it would be hard to start. A faulty ignition pickup can make it hard to start. Weaker than normal spark can make starting more difficult.
So yes, there are other reasons a motor can be hard to start. crankshaft seals when cold are contracted and when they heat up they expand and start to leak RV cavity oil into left or right or both sides of the case and into the fuel/air mix.
So, in conclusion, whether its oil, or gas, in the lower case, it will be hard to start.