A good practice when it comes to distributing
propane on a travel trailer is to run soft copper outside the trailer(underneath), and to make all connections that deliver
propane to an appliance, outside(underneath) the trailer. That way, all connections which can possibly leak , are visible and can be checked with soapy water or a combustible gas tester. If you only want
propane for your cooktop, I don’t see a problem with running the line inside as long as you are sure the line won’t be damaged by stored supplies, tools, etc.. no inside splices or plugged t- fittings in hidden out of site locations.
To use copper lines you will need a flaring tool and to take some time practicing using it. I suspect there are some good utube videos that would be helpful.