If the compressor is good, any refig. mechanic should be able to find the leak, fix it, and gas it.
You said "top and bottom" lines, the line from the compressor to the outside condenser coil should be getting warm and the line running from the condenser to the inside evaporator coil and the coil itself should get cold.
If you have the knowledge and tools to fix it I'm sure it would be cost effective.
Since it's been open since last year you'll need to pull a very good vacuum once it's repaired and add a dryer.
A new top unit here is about twice what the repair would cost.
If you have to pay someone else, the question is what will they charge you. If you take it off the camper and in to their shop they might be real reasonable. Can't hurt to ask.
You might find a used one for the cost of the repair.