Hello Dump Trucker: Sorry to hear the radio started to have problems and such.
Modifying the radio from the stock power levels to higher power levels may have caused the SRF indication of low or no RF Output. How did you measure the SWR with little or no RF Output?
I would think driving into the rising Sun shouldn't have caused that much heat to make the radio brake.
I can see a Dump Truck vibrating may cause problems.
How ever trouble shooting the radio and seeing what really happened will tell the real story. The modification is considered a possible problem as this seems to be the only thing touched in the radio. The bad thing is the amount of RF Output gain as compared to actual gain of increasing your range, is really NOT worth pushing the radios transmitter to the limits, causing reliability problems. The manufactures have little dudes running around wearing bow ties (bean counters) wanting to cut costs to increase the profit margin. They didn't do us any favors by designing and manufacturing the radios with expensive transmitter parts that can be easily modified to a "significant amount of RF Output Power in a reliable way. This condition since the start of the CB Radio manufacturing mass quantities of radios. Yeah some radios did have a extra head room for modifications....
18 watts increased to 40 watts PEP is only 3.5 dB gain, not noticeable. You need at least a 6 dB gain which would be 72 watts PEP, or better a 10dB gain at 180 watts. That's what small 150 to 250 watt amps are so popular. So running a stock radio with good audio into a small amp is way better.
Many years ago I bought a old 1974 Dodge Duster from a friend as a transportation car. Needed a tune up, battery, oil change, filters, and major cleaning but for 50 dollars American I couldn't complain, it even came with a little cheapie CB radio and M-125 antenna, great!
I owned the car for years and the little CB radio worked like a charm taking to the locals going thru the mountains and such, and even long distances. I found out later it had a small 50 watt amp installed in the trunk but had black under coating sprayed over it, making it difficult to remove. Thats why the radio worked so well! I sold the car years later to a guy who wanted the engine and trans.
Good Luck
Jay in the Mojave