Since this problem began after you added the relay for charging the camper battery, you might want to temporarily disconnect your modification, just in case it is loading down the battery and preventing the starter from getting enough current. I consider this an unlikely scenario, think you probably just have a "tired" solenoid as others have said, but it's worth eliminating that possibility while you're checking all the grounds and such.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .343,000 miles Wife's car
2005 E320 CDI . . 148,000 miles My car
Santa Rosa population 177,300 (2026 projected)
Total. . . . . . . . . . . . 668,300
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970