1986 Questions for all other cars Engine missing, temperatur

Hi,

I actually have two questions, about my 1986 Winnebago RV that has a chevy 454 engine in it.

The first problem is misfires. The vehicle sat with approximately 14 months with half a tank of gas in it. This spring I had it tuned up, timing, plugs, cables, etc. I didn't drain the old gas but I did top it off with premium and put in some additives. It ran fine for the first 100 or so miles. Then it started misfiring when I was going up hills. It started very subtlely, but soon progressed to where I was concerned about making it home. I was down to about 1/2 a tank of gas, so I topped it off with premium again, but it didn't help. It only misses under load. I did some research that said check the spark plug cables, but they are new. I was wondering if it perhaps was the fuel pump, or fuel filter. Any suggestions would be appreciated.

My second question. My temperature gauge doesn't work. I wasn't concerned about it until the other problem developed, and I've read that these engines have a tendency to run hot. I'm guessing it's not the gauge itself since the other functions on it are ok, so what component are there, and what do I need to look at? Am I better pursuing an aftermarket product, and if so, what would you recommend.

On an unrelated note, I'm happy to donate, this is a great site, but I couldn't find a suggested amount. If $10 isn't enough, please let me know. Thanks Jon