buckstrucks

If you are even remotely handy you can pick up a timing light and a wrench and set the ignition timing very quickly your self with a bit of help from you tube. A normal tune up involves replacing plugs, wires, setting timing and going about your day. Many people also change fuel and air filters along with an oil filter change at the same time. All of this can be easily done on your own with a very minimal investment in tools. I am not familiar with the carb your older motor home will have in it. Setting the adjustment screws for your elevation may help. But I am guessing an island is nearish to sea level. Are you experiencing any problems? Or is this just preventive maintenance?

If the heads are not stripped, do not drill them. You need to either pick up a good cordless impact gun (I like Makita) and a quality bit set (I like Milwaukee). Heat and oil several like you have been doing then tap several times with a heavy hammer, I use a 40oz snap on steel faced dead blow. Then attempt removal with the impact gun, easy does it, maintain firm pressure on the screw. If you are on a budget Harbor Freight sells impact drivers that you hit with a hammer and they rotate with the impact. These will break loose your screws, or strip them. But since you planned on drilling them anyways its worth the effort. Once you get one of the cordless impact guns your life will be changed. They do everything better than manual screwdrivers except fit in tight spaces. I basically use mine as a replacement for a ratchet if it will fit.