Our houseboat does this, it's simply due to resistance, caused by the large amounts of wiring used. Electric gauges use resistance to ground to move the needles. When more load is applied to the electric system and the voltage drops, resistance increases, and the gauges will read higher.
It IS normal because the lighting system eats a lot of power. Just not very desirable...
2000 40' Tiffin Allegro Bus \ 2002 Regal 2860 Commodore
1988 53 x 14 Skipperliner \ 1995 32' Party Cruiser
1984 Goldwing Aspencade \ 1976 Harley Sportster